Add native histogram support in federation

Signed-off-by: Ganesh Vernekar <ganeshvern@gmail.com>

web/federate.go

@@ -26,6 +26,7 @@ import (
 	"github.com/prometheus/common/expfmt"
 	"github.com/prometheus/common/model"
 
+	"github.com/prometheus/prometheus/model/histogram"
 	"github.com/prometheus/prometheus/model/labels"
 	"github.com/prometheus/prometheus/model/timestamp"
 	"github.com/prometheus/prometheus/model/value"
@@ -103,6 +104,7 @@ func (h *Handler) federation(w http.ResponseWriter, req *http.Request) {
 	set := storage.NewMergeSeriesSet(sets, storage.ChainedSeriesMerge)
 	it := storage.NewBuffer(int64(h.lookbackDelta / 1e6))
 	var chkIter chunkenc.Iterator
+Loop:
 	for set.Next() {
 		s := set.At()
 
@@ -111,18 +113,22 @@ func (h *Handler) federation(w http.ResponseWriter, req *http.Request) {
 		chkIter = s.Iterator(chkIter)
 		it.Reset(chkIter)
 
-		var t int64
-		var v float64
-		var ok bool
-
+		var (
+			t  int64
+			v  float64
+			h  *histogram.FloatHistogram
+			ok bool
+		)
 		valueType := it.Seek(maxt)
-		if valueType == chunkenc.ValFloat {
+		switch valueType {
+		case chunkenc.ValFloat:
 			t, v = it.At()
-		} else {
-			// TODO(beorn7): Handle histograms.
+		case chunkenc.ValFloatHistogram, chunkenc.ValHistogram:
+			t, h = it.AtFloatHistogram()
+		default:
 			t, v, _, ok = it.PeekBack(1)
 			if !ok {
-				continue
+				continue Loop
 			}
 		}
 		// The exposition formats do not support stale markers, so drop them. This
@@ -135,7 +141,7 @@ func (h *Handler) federation(w http.ResponseWriter, req *http.Request) {
 
 		vec = append(vec, promql.Sample{
 			Metric: s.Labels(),
-			Point:  promql.Point{T: t, V: v},
+			Point:  promql.Point{T: t, V: v, H: h},
 		})
 	}
 	if ws := set.Warnings(); len(ws) > 0 {
@@ -162,14 +168,21 @@ func (h *Handler) federation(w http.ResponseWriter, req *http.Request) {
 
 	var (
 		lastMetricName                          string
+		lastWasHistogram, lastHistogramWasGauge bool
 		protMetricFam                           *dto.MetricFamily
 	)
 	for _, s := range vec {
+		isHistogram := s.H != nil
+		if isHistogram &&
+			format != expfmt.FmtProtoDelim && format != expfmt.FmtProtoText && format != expfmt.FmtProtoCompact {
+			// Can't serve the native histogram.
+			// TODO(codesome): Serve them when other protocols get the native histogram support.
+			continue
+		}
+
 		nameSeen := false
 		globalUsed := map[string]struct{}{}
-		protMetric := &dto.Metric{
-			Untyped: &dto.Untyped{},
-		}
+		protMetric := &dto.Metric{}
 
 		err := s.Metric.Validate(func(l labels.Label) error {
 			if l.Value == "" {
@@ -179,11 +192,18 @@ func (h *Handler) federation(w http.ResponseWriter, req *http.Request) {
 			}
 			if l.Name == labels.MetricName {
 				nameSeen = true
-				if l.Value == lastMetricName {
-					// We already have the name in the current MetricFamily,
-					// and we ignore nameless metrics.
+				if l.Value == lastMetricName && // We already have the name in the current MetricFamily, and we ignore nameless metrics.
+					lastWasHistogram == isHistogram && // The sample type matches (float vs histogram).
+					// If it was a histogram, the histogram type (counter vs gauge) also matches.
+					(!isHistogram || lastHistogramWasGauge == (s.H.CounterResetHint == histogram.GaugeType)) {
 					return nil
 				}
+
+				// Since we now check for the sample type and type of histogram above, we will end up
+				// creating multiple metric families for the same metric name. This would technically be
+				// an invalid exposition. But since the consumer of this is Prometheus, and Prometheus can
+				// parse it fine, we allow it and bend the rules to make federation possible in those cases.
+
 				// Need to start a new MetricFamily. Ship off the old one (if any) before
 				// creating the new one.
 				if protMetricFam != nil {
@@ -195,6 +215,13 @@ func (h *Handler) federation(w http.ResponseWriter, req *http.Request) {
 					Type: dto.MetricType_UNTYPED.Enum(),
 					Name: proto.String(l.Value),
 				}
+				if isHistogram {
+					if s.H.CounterResetHint == histogram.GaugeType {
+						protMetricFam.Type = dto.MetricType_GAUGE_HISTOGRAM.Enum()
+					} else {
+						protMetricFam.Type = dto.MetricType_HISTOGRAM.Enum()
+					}
+				}
 				lastMetricName = l.Value
 				return nil
 			}
@@ -228,9 +255,42 @@ func (h *Handler) federation(w http.ResponseWriter, req *http.Request) {
 		}
 
 		protMetric.TimestampMs = proto.Int64(s.T)
-		protMetric.Untyped.Value = proto.Float64(s.V)
-		// TODO(beorn7): Handle histograms.
-
+		if !isHistogram {
+			lastHistogramWasGauge = false
+			protMetric.Untyped = &dto.Untyped{
+				Value: proto.Float64(s.V),
+			}
+		} else {
+			lastHistogramWasGauge = s.H.CounterResetHint == histogram.GaugeType
+			protMetric.Histogram = &dto.Histogram{
+				SampleCountFloat: proto.Float64(s.H.Count),
+				SampleSum:        proto.Float64(s.H.Sum),
+				Schema:           proto.Int32(s.H.Schema),
+				ZeroThreshold:    proto.Float64(s.H.ZeroThreshold),
+				ZeroCountFloat:   proto.Float64(s.H.ZeroCount),
+				NegativeCount:    s.H.NegativeBuckets,
+				PositiveCount:    s.H.PositiveBuckets,
+			}
+			if len(s.H.PositiveSpans) > 0 {
+				protMetric.Histogram.PositiveSpan = make([]*dto.BucketSpan, len(s.H.PositiveSpans))
+				for i, sp := range s.H.PositiveSpans {
+					protMetric.Histogram.PositiveSpan[i] = &dto.BucketSpan{
+						Offset: proto.Int32(sp.Offset),
+						Length: proto.Uint32(sp.Length),
+					}
+				}
+			}
+			if len(s.H.NegativeSpans) > 0 {
+				protMetric.Histogram.NegativeSpan = make([]*dto.BucketSpan, len(s.H.NegativeSpans))
+				for i, sp := range s.H.NegativeSpans {
+					protMetric.Histogram.NegativeSpan[i] = &dto.BucketSpan{
+						Offset: proto.Int32(sp.Offset),
+						Length: proto.Uint32(sp.Length),
+					}
+				}
+			}
+		}
+		lastWasHistogram = isHistogram
 		protMetricFam.Metric = append(protMetricFam.Metric, protMetric)
 	}
 	// Still have to ship off the last MetricFamily, if any.