diff --git a/prompb/custom.go b/prompb/custom.go
index 7f97b2734b..613338b3eb 100644
--- a/prompb/custom.go
+++ b/prompb/custom.go
@@ -14,8 +14,9 @@
 package prompb
 
 import (
-	"slices"
 	"sync"
+
+	"golang.org/x/exp/slices"
 )
 
 func (m Sample) T() int64   { return m.Timestamp }
diff --git a/prompb/custom_test.go b/prompb/custom_test.go
index b12f49a338..cddae1a005 100644
--- a/prompb/custom_test.go
+++ b/prompb/custom_test.go
@@ -1,3 +1,15 @@
+// Copyright 2023 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.
 package prompb
 
 import (