diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index e0f5cdf660..d6d79b6fc3 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -17,6 +17,7 @@ jobs:
       - uses: ./.github/promci/actions/setup_environment
       - run: make GO_ONLY=1 SKIP_GOLANGCI_LINT=1
       - run: go test ./tsdb/ -test.tsdb-isolation=false
+      - run: go test --tags=stringlabels ./...
       - run: GOARCH=386 go test ./cmd/prometheus
       - run: make -C documentation/examples/remote_storage
       - run: make -C documentation/examples
diff --git a/model/labels/labels.go b/model/labels/labels.go
index 36a0e6cb35..18b8f8dc62 100644
--- a/model/labels/labels.go
+++ b/model/labels/labels.go
@@ -11,6 +11,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+//go:build !stringlabels
+
 package labels
 
 import (
diff --git a/model/labels/labels_string.go b/model/labels/labels_string.go
new file mode 100644
index 0000000000..e912882fe3
--- /dev/null
+++ b/model/labels/labels_string.go
@@ -0,0 +1,787 @@
+// Copyright 2017 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//go:build stringlabels
+
+package labels
+
+import (
+	"bytes"
+	"encoding/json"
+	"reflect"
+	"sort"
+	"strconv"
+	"unsafe"
+
+	"github.com/cespare/xxhash/v2"
+	"github.com/prometheus/common/model"
+)
+
+// Well-known label names used by Prometheus components.
+const (
+	MetricName   = "__name__"
+	AlertName    = "alertname"
+	BucketLabel  = "le"
+	InstanceName = "instance"
+)
+
+var seps = []byte{'\xff'}
+
+// Label is a key/value pair of strings.
+type Label struct {
+	Name, Value string
+}
+
+// Labels is implemented by a single flat string holding name/value pairs.
+// Each name and value is preceded by its length in varint encoding.
+// Names are in order.
+type Labels struct {
+	data string
+}
+
+type labelSlice []Label
+
+func (ls labelSlice) Len() int           { return len(ls) }
+func (ls labelSlice) Swap(i, j int)      { ls[i], ls[j] = ls[j], ls[i] }
+func (ls labelSlice) Less(i, j int) bool { return ls[i].Name < ls[j].Name }
+
+func decodeSize(data string, index int) (int, int) {
+	var size int
+	for shift := uint(0); ; shift += 7 {
+		// Just panic if we go of the end of data, since all Labels strings are constructed internally and
+		// malformed data indicates a bug, or memory corruption.
+		b := data[index]
+		index++
+		size |= int(b&0x7F) << shift
+		if b < 0x80 {
+			break
+		}
+	}
+	return size, index
+}
+
+func decodeString(data string, index int) (string, int) {
+	var size int
+	size, index = decodeSize(data, index)
+	return data[index : index+size], index + size
+}
+
+func (ls Labels) String() string {
+	var b bytes.Buffer
+
+	b.WriteByte('{')
+	for i := 0; i < len(ls.data); {
+		if i > 0 {
+			b.WriteByte(',')
+			b.WriteByte(' ')
+		}
+		var name, value string
+		name, i = decodeString(ls.data, i)
+		value, i = decodeString(ls.data, i)
+		b.WriteString(name)
+		b.WriteByte('=')
+		b.WriteString(strconv.Quote(value))
+	}
+	b.WriteByte('}')
+	return b.String()
+}
+
+// Bytes returns ls as a byte slice.
+// It uses non-printing characters and so should not be used for printing.
+func (ls Labels) Bytes(buf []byte) []byte {
+	if cap(buf) < len(ls.data) {
+		buf = make([]byte, len(ls.data))
+	} else {
+		buf = buf[:len(ls.data)]
+	}
+	copy(buf, ls.data)
+	return buf
+}
+
+// MarshalJSON implements json.Marshaler.
+func (ls Labels) MarshalJSON() ([]byte, error) {
+	return json.Marshal(ls.Map())
+}
+
+// UnmarshalJSON implements json.Unmarshaler.
+func (ls *Labels) UnmarshalJSON(b []byte) error {
+	var m map[string]string
+
+	if err := json.Unmarshal(b, &m); err != nil {
+		return err
+	}
+
+	*ls = FromMap(m)
+	return nil
+}
+
+// MarshalYAML implements yaml.Marshaler.
+func (ls Labels) MarshalYAML() (interface{}, error) {
+	return ls.Map(), nil
+}
+
+// IsZero implements yaml.IsZeroer - if we don't have this then 'omitempty' fields are always omitted.
+func (ls Labels) IsZero() bool {
+	return len(ls.data) == 0
+}
+
+// UnmarshalYAML implements yaml.Unmarshaler.
+func (ls *Labels) UnmarshalYAML(unmarshal func(interface{}) error) error {
+	var m map[string]string
+
+	if err := unmarshal(&m); err != nil {
+		return err
+	}
+
+	*ls = FromMap(m)
+	return nil
+}
+
+// MatchLabels returns a subset of Labels that matches/does not match with the provided label names based on the 'on' boolean.
+// If on is set to true, it returns the subset of labels that match with the provided label names and its inverse when 'on' is set to false.
+// TODO: This is only used in printing an error message
+func (ls Labels) MatchLabels(on bool, names ...string) Labels {
+	b := NewBuilder(ls)
+	if on {
+		b.Keep(names...)
+	} else {
+		b.Del(MetricName)
+		b.Del(names...)
+	}
+	return b.Labels(EmptyLabels())
+}
+
+// Hash returns a hash value for the label set.
+// Note: the result is not guaranteed to be consistent across different runs of Prometheus.
+func (ls Labels) Hash() uint64 {
+	return xxhash.Sum64(yoloBytes(ls.data))
+}
+
+// HashForLabels returns a hash value for the labels matching the provided names.
+// 'names' have to be sorted in ascending order.
+func (ls Labels) HashForLabels(b []byte, names ...string) (uint64, []byte) {
+	b = b[:0]
+	j := 0
+	for i := 0; i < len(ls.data); {
+		var name, value string
+		name, i = decodeString(ls.data, i)
+		value, i = decodeString(ls.data, i)
+		for j < len(names) && names[j] < name {
+			j++
+		}
+		if j == len(names) {
+			break
+		}
+		if name == names[j] {
+			b = append(b, name...)
+			b = append(b, seps[0])
+			b = append(b, value...)
+			b = append(b, seps[0])
+		}
+	}
+
+	return xxhash.Sum64(b), b
+}
+
+// HashWithoutLabels returns a hash value for all labels except those matching
+// the provided names.
+// 'names' have to be sorted in ascending order.
+func (ls Labels) HashWithoutLabels(b []byte, names ...string) (uint64, []byte) {
+	b = b[:0]
+	j := 0
+	for i := 0; i < len(ls.data); {
+		var name, value string
+		name, i = decodeString(ls.data, i)
+		value, i = decodeString(ls.data, i)
+		for j < len(names) && names[j] < name {
+			j++
+		}
+		if name == MetricName || (j < len(names) && name == names[j]) {
+			continue
+		}
+		b = append(b, name...)
+		b = append(b, seps[0])
+		b = append(b, value...)
+		b = append(b, seps[0])
+	}
+	return xxhash.Sum64(b), b
+}
+
+// BytesWithLabels is just as Bytes(), but only for labels matching names.
+// 'names' have to be sorted in ascending order.
+func (ls Labels) BytesWithLabels(buf []byte, names ...string) []byte {
+	b := buf[:0]
+	j := 0
+	for pos := 0; pos < len(ls.data); {
+		lName, newPos := decodeString(ls.data, pos)
+		_, newPos = decodeString(ls.data, newPos)
+		for j < len(names) && names[j] < lName {
+			j++
+		}
+		if j == len(names) {
+			break
+		}
+		if lName == names[j] {
+			b = append(b, ls.data[pos:newPos]...)
+		}
+		pos = newPos
+	}
+	return b
+}
+
+// BytesWithoutLabels is just as Bytes(), but only for labels not matching names.
+// 'names' have to be sorted in ascending order.
+func (ls Labels) BytesWithoutLabels(buf []byte, names ...string) []byte {
+	b := buf[:0]
+	j := 0
+	for pos := 0; pos < len(ls.data); {
+		lName, newPos := decodeString(ls.data, pos)
+		_, newPos = decodeString(ls.data, newPos)
+		for j < len(names) && names[j] < lName {
+			j++
+		}
+		if j == len(names) || lName != names[j] {
+			b = append(b, ls.data[pos:newPos]...)
+		}
+		pos = newPos
+	}
+	return b
+}
+
+// Copy returns a copy of the labels.
+func (ls Labels) Copy() Labels {
+	buf := append([]byte{}, ls.data...)
+	return Labels{data: yoloString(buf)}
+}
+
+// Get returns the value for the label with the given name.
+// Returns an empty string if the label doesn't exist.
+func (ls Labels) Get(name string) string {
+	for i := 0; i < len(ls.data); {
+		var lName, lValue string
+		lName, i = decodeString(ls.data, i)
+		lValue, i = decodeString(ls.data, i)
+		if lName == name {
+			return lValue
+		}
+	}
+	return ""
+}
+
+// Has returns true if the label with the given name is present.
+func (ls Labels) Has(name string) bool {
+	for i := 0; i < len(ls.data); {
+		var lName string
+		lName, i = decodeString(ls.data, i)
+		_, i = decodeString(ls.data, i)
+		if lName == name {
+			return true
+		}
+	}
+	return false
+}
+
+// HasDuplicateLabelNames returns whether ls has duplicate label names.
+// It assumes that the labelset is sorted.
+func (ls Labels) HasDuplicateLabelNames() (string, bool) {
+	var lName, prevName string
+	for i := 0; i < len(ls.data); {
+		lName, i = decodeString(ls.data, i)
+		_, i = decodeString(ls.data, i)
+		if lName == prevName {
+			return lName, true
+		}
+		prevName = lName
+	}
+	return "", false
+}
+
+// WithoutEmpty returns the labelset without empty labels.
+// May return the same labelset.
+func (ls Labels) WithoutEmpty() Labels {
+	for pos := 0; pos < len(ls.data); {
+		_, newPos := decodeString(ls.data, pos)
+		lValue, newPos := decodeString(ls.data, newPos)
+		if lValue != "" {
+			pos = newPos
+			continue
+		}
+		// Do not copy the slice until it's necessary.
+		// TODO: could optimise the case where all blanks are at the end.
+		// Note: we size the new buffer on the assumption there is exactly one blank value.
+		buf := make([]byte, pos, pos+(len(ls.data)-newPos))
+		copy(buf, ls.data[:pos]) // copy the initial non-blank labels
+		pos = newPos             // move past the first blank value
+		for pos < len(ls.data) {
+			var newPos int
+			_, newPos = decodeString(ls.data, pos)
+			lValue, newPos = decodeString(ls.data, newPos)
+			if lValue != "" {
+				buf = append(buf, ls.data[pos:newPos]...)
+			}
+			pos = newPos
+		}
+		return Labels{data: yoloString(buf)}
+	}
+	return ls
+}
+
+// IsValid checks if the metric name or label names are valid.
+func (ls Labels) IsValid() bool {
+	err := ls.Validate(func(l Label) error {
+		if l.Name == model.MetricNameLabel && !model.IsValidMetricName(model.LabelValue(l.Value)) {
+			return strconv.ErrSyntax
+		}
+		if !model.LabelName(l.Name).IsValid() || !model.LabelValue(l.Value).IsValid() {
+			return strconv.ErrSyntax
+		}
+		return nil
+	})
+	return err == nil
+}
+
+// Equal returns whether the two label sets are equal.
+func Equal(ls, o Labels) bool {
+	return ls.data == o.data
+}
+
+// Map returns a string map of the labels.
+func (ls Labels) Map() map[string]string {
+	m := make(map[string]string, len(ls.data)/10)
+	for i := 0; i < len(ls.data); {
+		var lName, lValue string
+		lName, i = decodeString(ls.data, i)
+		lValue, i = decodeString(ls.data, i)
+		m[lName] = lValue
+	}
+	return m
+}
+
+// EmptyLabels returns an empty Labels value, for convenience.
+func EmptyLabels() Labels {
+	return Labels{}
+}
+
+func yoloString(b []byte) string {
+	return *((*string)(unsafe.Pointer(&b)))
+}
+
+func yoloBytes(s string) (b []byte) {
+	*(*string)(unsafe.Pointer(&b)) = s
+	(*reflect.SliceHeader)(unsafe.Pointer(&b)).Cap = len(s)
+	return
+}
+
+// New returns a sorted Labels from the given labels.
+// The caller has to guarantee that all label names are unique.
+func New(ls ...Label) Labels {
+	sort.Sort(labelSlice(ls))
+	size := labelsSize(ls)
+	buf := make([]byte, size)
+	marshalLabelsToSizedBuffer(ls, buf)
+	return Labels{data: yoloString(buf)}
+}
+
+// FromMap returns new sorted Labels from the given map.
+func FromMap(m map[string]string) Labels {
+	l := make([]Label, 0, len(m))
+	for k, v := range m {
+		l = append(l, Label{Name: k, Value: v})
+	}
+	return New(l...)
+}
+
+// FromStrings creates new labels from pairs of strings.
+func FromStrings(ss ...string) Labels {
+	if len(ss)%2 != 0 {
+		panic("invalid number of strings")
+	}
+	ls := make([]Label, 0, len(ss)/2)
+	for i := 0; i < len(ss); i += 2 {
+		ls = append(ls, Label{Name: ss[i], Value: ss[i+1]})
+	}
+
+	sort.Sort(labelSlice(ls))
+	return New(ls...)
+}
+
+// Compare compares the two label sets.
+// The result will be 0 if a==b, <0 if a < b, and >0 if a > b.
+// TODO: replace with Less function - Compare is never needed.
+// TODO: just compare the underlying strings when we don't need alphanumeric sorting.
+func Compare(a, b Labels) int {
+	l := len(a.data)
+	if len(b.data) < l {
+		l = len(b.data)
+	}
+
+	ia, ib := 0, 0
+	for ia < l {
+		var aName, bName string
+		aName, ia = decodeString(a.data, ia)
+		bName, ib = decodeString(b.data, ib)
+		if aName != bName {
+			if aName < bName {
+				return -1
+			}
+			return 1
+		}
+		var aValue, bValue string
+		aValue, ia = decodeString(a.data, ia)
+		bValue, ib = decodeString(b.data, ib)
+		if aValue != bValue {
+			if aValue < bValue {
+				return -1
+			}
+			return 1
+		}
+	}
+	// If all labels so far were in common, the set with fewer labels comes first.
+	return len(a.data) - len(b.data)
+}
+
+// Copy labels from b on top of whatever was in ls previously, reusing memory or expanding if needed.
+func (ls *Labels) CopyFrom(b Labels) {
+	ls.data = b.data // strings are immutable
+}
+
+// IsEmpty returns true if ls represents an empty set of labels.
+func (ls Labels) IsEmpty() bool {
+	return len(ls.data) == 0
+}
+
+// Len returns the number of labels; it is relatively slow.
+func (ls Labels) Len() int {
+	count := 0
+	for i := 0; i < len(ls.data); {
+		var size int
+		size, i = decodeSize(ls.data, i)
+		i += size
+		size, i = decodeSize(ls.data, i)
+		i += size
+		count++
+	}
+	return count
+}
+
+// Range calls f on each label.
+func (ls Labels) Range(f func(l Label)) {
+	for i := 0; i < len(ls.data); {
+		var lName, lValue string
+		lName, i = decodeString(ls.data, i)
+		lValue, i = decodeString(ls.data, i)
+		f(Label{Name: lName, Value: lValue})
+	}
+}
+
+// Validate calls f on each label. If f returns a non-nil error, then it returns that error cancelling the iteration.
+func (ls Labels) Validate(f func(l Label) error) error {
+	for i := 0; i < len(ls.data); {
+		var lName, lValue string
+		lName, i = decodeString(ls.data, i)
+		lValue, i = decodeString(ls.data, i)
+		err := f(Label{Name: lName, Value: lValue})
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// InternStrings calls intern on every string value inside ls, replacing them with what it returns.
+func (ls *Labels) InternStrings(intern func(string) string) {
+	ls.data = intern(ls.data)
+}
+
+// ReleaseStrings calls release on every string value inside ls.
+func (ls Labels) ReleaseStrings(release func(string)) {
+	release(ls.data)
+}
+
+// Builder allows modifying Labels.
+type Builder struct {
+	base Labels
+	del  []string
+	add  []Label
+}
+
+// NewBuilder returns a new LabelsBuilder.
+func NewBuilder(base Labels) *Builder {
+	b := &Builder{
+		del: make([]string, 0, 5),
+		add: make([]Label, 0, 5),
+	}
+	b.Reset(base)
+	return b
+}
+
+// Reset clears all current state for the builder.
+func (b *Builder) Reset(base Labels) {
+	b.base = base
+	b.del = b.del[:0]
+	b.add = b.add[:0]
+	for i := 0; i < len(base.data); {
+		var lName, lValue string
+		lName, i = decodeString(base.data, i)
+		lValue, i = decodeString(base.data, i)
+		if lValue == "" {
+			b.del = append(b.del, lName)
+		}
+	}
+}
+
+// Del deletes the label of the given name.
+func (b *Builder) Del(ns ...string) *Builder {
+	for _, n := range ns {
+		for i, a := range b.add {
+			if a.Name == n {
+				b.add = append(b.add[:i], b.add[i+1:]...)
+			}
+		}
+		b.del = append(b.del, n)
+	}
+	return b
+}
+
+// Keep removes all labels from the base except those with the given names.
+func (b *Builder) Keep(ns ...string) *Builder {
+Outer:
+	for i := 0; i < len(b.base.data); {
+		var lName string
+		lName, i = decodeString(b.base.data, i)
+		_, i = decodeString(b.base.data, i)
+		for _, n := range ns {
+			if lName == n {
+				continue Outer
+			}
+		}
+		b.del = append(b.del, lName)
+	}
+	return b
+}
+
+// Set the name/value pair as a label. A value of "" means delete that label.
+func (b *Builder) Set(n, v string) *Builder {
+	if v == "" {
+		// Empty labels are the same as missing labels.
+		return b.Del(n)
+	}
+	for i, a := range b.add {
+		if a.Name == n {
+			b.add[i].Value = v
+			return b
+		}
+	}
+	b.add = append(b.add, Label{Name: n, Value: v})
+
+	return b
+}
+
+// Labels returns the labels from the builder, adding them to res if non-nil.
+// Argument res can be the same as b.base, if caller wants to overwrite that slice.
+// If no modifications were made, the original labels are returned.
+func (b *Builder) Labels(res Labels) Labels {
+	if len(b.del) == 0 && len(b.add) == 0 {
+		return b.base
+	}
+
+	sort.Sort(labelSlice(b.add))
+	sort.Strings(b.del)
+	a, d := 0, 0
+
+	buf := make([]byte, 0, len(b.base.data)) // TODO: see if we can re-use the buffer from res.
+	for pos := 0; pos < len(b.base.data); {
+		oldPos := pos
+		var lName string
+		lName, pos = decodeString(b.base.data, pos)
+		_, pos = decodeString(b.base.data, pos)
+		for d < len(b.del) && b.del[d] < lName {
+			d++
+		}
+		if d < len(b.del) && b.del[d] == lName {
+			continue // This label has been deleted.
+		}
+		for ; a < len(b.add) && b.add[a].Name < lName; a++ {
+			buf = appendLabelTo(buf, &b.add[a]) // Insert label that was not in the base set.
+		}
+		if a < len(b.add) && b.add[a].Name == lName {
+			buf = appendLabelTo(buf, &b.add[a])
+			a++
+			continue // This label has been replaced.
+		}
+		buf = append(buf, b.base.data[oldPos:pos]...)
+	}
+	// We have come to the end of the base set; add any remaining labels.
+	for ; a < len(b.add); a++ {
+		buf = appendLabelTo(buf, &b.add[a])
+	}
+	return Labels{data: yoloString(buf)}
+}
+
+func marshalLabelsToSizedBuffer(lbls []Label, data []byte) int {
+	i := len(data)
+	for index := len(lbls) - 1; index >= 0; index-- {
+		size := marshalLabelToSizedBuffer(&lbls[index], data[:i])
+		i -= size
+	}
+	return len(data) - i
+}
+
+func marshalLabelToSizedBuffer(m *Label, data []byte) int {
+	i := len(data)
+	i -= len(m.Value)
+	copy(data[i:], m.Value)
+	i = encodeSize(data, i, len(m.Value))
+	i -= len(m.Name)
+	copy(data[i:], m.Name)
+	i = encodeSize(data, i, len(m.Name))
+	return len(data) - i
+}
+
+func sizeVarint(x uint64) (n int) {
+	// Most common case first
+	if x < 1<<7 {
+		return 1
+	}
+	if x >= 1<<56 {
+		return 9
+	}
+	if x >= 1<<28 {
+		x >>= 28
+		n = 4
+	}
+	if x >= 1<<14 {
+		x >>= 14
+		n += 2
+	}
+	if x >= 1<<7 {
+		n++
+	}
+	return n + 1
+}
+
+func encodeVarint(data []byte, offset int, v uint64) int {
+	offset -= sizeVarint(v)
+	base := offset
+	for v >= 1<<7 {
+		data[offset] = uint8(v&0x7f | 0x80)
+		v >>= 7
+		offset++
+	}
+	data[offset] = uint8(v)
+	return base
+}
+
+// Special code for the common case that a size is less than 128
+func encodeSize(data []byte, offset, v int) int {
+	if v < 1<<7 {
+		offset--
+		data[offset] = uint8(v)
+		return offset
+	}
+	return encodeVarint(data, offset, uint64(v))
+}
+
+func labelsSize(lbls []Label) (n int) {
+	// we just encode name/value/name/value, without any extra tags or length bytes
+	for _, e := range lbls {
+		n += labelSize(&e)
+	}
+	return n
+}
+
+func labelSize(m *Label) (n int) {
+	// strings are encoded as length followed by contents.
+	l := len(m.Name)
+	n += l + sizeVarint(uint64(l))
+	l = len(m.Value)
+	n += l + sizeVarint(uint64(l))
+	return n
+}
+
+func appendLabelTo(buf []byte, m *Label) []byte {
+	size := labelSize(m)
+	sizeRequired := len(buf) + size
+	if cap(buf) >= sizeRequired {
+		buf = buf[:sizeRequired]
+	} else {
+		bufSize := cap(buf)
+		// Double size of buffer each time it needs to grow, to amortise copying cost.
+		for bufSize < sizeRequired {
+			bufSize = bufSize*2 + 1
+		}
+		newBuf := make([]byte, sizeRequired, bufSize)
+		copy(newBuf, buf)
+		buf = newBuf
+	}
+	marshalLabelToSizedBuffer(m, buf)
+	return buf
+}
+
+// ScratchBuilder allows efficient construction of a Labels from scratch.
+type ScratchBuilder struct {
+	add             []Label
+	output          Labels
+	overwriteBuffer []byte
+}
+
+// NewScratchBuilder creates a ScratchBuilder initialized for Labels with n entries.
+func NewScratchBuilder(n int) ScratchBuilder {
+	return ScratchBuilder{add: make([]Label, 0, n)}
+}
+
+func (b *ScratchBuilder) Reset() {
+	b.add = b.add[:0]
+	b.output = EmptyLabels()
+}
+
+// Add a name/value pair.
+// Note if you Add the same name twice you will get a duplicate label, which is invalid.
+func (b *ScratchBuilder) Add(name, value string) {
+	b.add = append(b.add, Label{Name: name, Value: value})
+}
+
+// Sort the labels added so far by name.
+func (b *ScratchBuilder) Sort() {
+	sort.Sort(labelSlice(b.add))
+}
+
+// Asssign is for when you already have a Labels which you want this ScratchBuilder to return.
+func (b *ScratchBuilder) Assign(l Labels) {
+	b.output = l
+}
+
+// Labels returns the name/value pairs added as a Labels object. Calling Add() after Labels() has no effect.
+// Note: if you want them sorted, call Sort() first.
+func (b *ScratchBuilder) Labels() Labels {
+	if b.output.IsEmpty() {
+		size := labelsSize(b.add)
+		buf := make([]byte, size)
+		marshalLabelsToSizedBuffer(b.add, buf)
+		b.output = Labels{data: yoloString(buf)}
+	}
+	return b.output
+}
+
+// Write the newly-built Labels out to ls, reusing an internal buffer.
+// Callers must ensure that there are no other references to ls.
+func (b *ScratchBuilder) Overwrite(ls *Labels) {
+	size := labelsSize(b.add)
+	if size <= cap(b.overwriteBuffer) {
+		b.overwriteBuffer = b.overwriteBuffer[:size]
+	} else {
+		b.overwriteBuffer = make([]byte, size)
+	}
+	marshalLabelsToSizedBuffer(b.add, b.overwriteBuffer)
+	ls.data = yoloString(b.overwriteBuffer)
+}
diff --git a/rules/alerting_test.go b/rules/alerting_test.go
index 057510e34d..b33c975fc1 100644
--- a/rules/alerting_test.go
+++ b/rules/alerting_test.go
@@ -919,8 +919,8 @@ func TestAlertingEvalWithOrigin(t *testing.T) {
 		time.Second,
 		time.Minute,
 		lbs,
-		nil,
-		nil,
+		labels.EmptyLabels(),
+		labels.EmptyLabels(),
 		"",
 		true, log.NewNopLogger(),
 	)