Add values to correct bucket

Signed-off-by: William Perron <william.perron@shopify.com>

prometheus/examples_test.go

@@ -16,13 +16,14 @@ package prometheus_test
 import (
 	"bytes"
 	"fmt"
-	"google.golang.org/protobuf/types/known/timestamppb"
 	"math"
 	"net/http"
 	"runtime"
 	"strings"
 	"time"
 
+	"google.golang.org/protobuf/types/known/timestamppb"
+
 	//nolint:staticcheck // Ignore SA1019. Need to keep deprecated package for compatibility.
 	"github.com/golang/protobuf/proto"
 	"github.com/prometheus/common/expfmt"
@@ -556,12 +557,12 @@ func ExampleNewConstHistogram() {
 	lp := dto.LabelPair{Name: &n, Value: &v}
 	var labelPairs []*dto.LabelPair
 	labelPairs = append(labelPairs, &lp)
-	val := float64(42)
+	vals := []float64{24.0, 42.0, 89.0, 157.0}
 	t, _ := time.Parse("unix", "Mon Jan _2 15:04:05 MST 2006")
 	ts := timestamppb.New(t)
 
 	for i := 0; i < 4; i++ {
-		e := dto.Exemplar{Label: labelPairs, Value: &val, Timestamp: ts}
+		e := dto.Exemplar{Label: labelPairs, Value: &vals[i], Timestamp: ts}
 		exemplars = append(exemplars, &e)
 	}
 
@@ -605,7 +606,7 @@ func ExampleNewConstHistogram() {
 	//         name: "testName"
 	//         value: "testVal"
 	//       >
-	//       value: 42
+	//       value: 24
 	//       timestamp: <
 	//         seconds: -62135596800
 	//       >
@@ -633,7 +634,7 @@ func ExampleNewConstHistogram() {
 	//         name: "testName"
 	//         value: "testVal"
 	//       >
-	//       value: 42
+	//       value: 89
 	//       timestamp: <
 	//         seconds: -62135596800
 	//       >
@@ -647,7 +648,7 @@ func ExampleNewConstHistogram() {
 	//         name: "testName"
 	//         value: "testVal"
 	//       >
-	//       value: 42
+	//       value: 157
 	//       timestamp: <
 	//         seconds: -62135596800
 	//       >

prometheus/histogram.go

@@ -582,7 +582,9 @@ func (h *constHistogram) Desc() *Desc {
 
 func (h *constHistogram) Write(out *dto.Metric) error {
 	his := &dto.Histogram{}
+	// h.buckets, buckets and bounds are all the same length
 	buckets := make([]*dto.Bucket, 0, len(h.buckets))
+	bounds := make([]float64, 0)
 
 	his.SampleCount = proto.Uint64(h.count)
 	his.SampleSum = proto.Float64(h.sum)
@@ -591,15 +593,24 @@ func (h *constHistogram) Write(out *dto.Metric) error {
 			CumulativeCount: proto.Uint64(count),
 			UpperBound:      proto.Float64(upperBound),
 		})
+		bounds = append(bounds, upperBound)
 	}
 
+	// make sure that both bounds and buckets have the same ordering
 	if len(buckets) > 0 {
 		sort.Sort(buckSort(buckets))
 	}
+	sort.Float64s(bounds)
 
 	if len(h.exemplars) > 0 {
-		for i := 0; i < len(buckets); i++ {
-			buckets[i].Exemplar = h.exemplars[i]
+		r := len(buckets)
+		for i := 0; i < r; i++ {
+			bound := sort.SearchFloat64s(bounds, *h.exemplars[i].Value)
+			// Only append the exemplar if it's within the bounds defined in the
+			// buckets.
+			if bound < r {
+				buckets[bound].Exemplar = h.exemplars[i]
+			}
 		}
 	}