Add support for exemplars on constHistogram

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

Author: arun-shopify

prometheus/examples_test.go

@@ -16,6 +16,7 @@ package prometheus_test
 import (
 "bytes"
 "fmt"
+"google.golang.org/protobuf/types/known/timestamppb"
 "math"
 "net/http"
 "runtime"
@@ -549,11 +550,27 @@ func ExampleNewConstHistogram() {
 prometheus.Labels{"owner": "example"},
 )
 
+var exemplars []*dto.Exemplar
+n := "testName"
+v := "testVal"
+lp := dto.LabelPair{Name: &n, Value: &v}
+var labelPairs []*dto.LabelPair
+labelPairs = append(labelPairs, &lp)
+val := float64(42)
+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}
+exemplars = append(exemplars, &e)
+}
+
 // Create a constant histogram from values we got from a 3rd party telemetry system.
-h := prometheus.MustNewConstHistogram(
+h := prometheus.MustNewConstHistogramWithExemplar(
 desc,
 4711, 403.34,
 map[float64]uint64{25: 121, 50: 2403, 100: 3221, 200: 4233},
+exemplars,
 "200", "get",
 )
 
@@ -583,18 +600,58 @@ func ExampleNewConstHistogram() {
 // bucket: <
 // cumulative_count: 121
 // upper_bound: 25
+// exemplar: <
+// label: <
+// name: "testName"
+// value: "testVal"
+// >
+// value: 42
+// timestamp: <
+// seconds: -62135596800
+// >
+// >
 // >
 // bucket: <
 // cumulative_count: 2403
 // upper_bound: 50
+// exemplar: <
+// label: <
+// name: "testName"
+// value: "testVal"
+// >
+// value: 42
+// timestamp: <
+// seconds: -62135596800
+// >
+// >
 // >
 // bucket: <
 // cumulative_count: 3221
 // upper_bound: 100
+// exemplar: <
+// label: <
+// name: "testName"
+// value: "testVal"
+// >
+// value: 42
+// timestamp: <
+// seconds: -62135596800
+// >
+// >
 // >
 // bucket: <
 // cumulative_count: 4233
 // upper_bound: 200
+// exemplar: <
+// label: <
+// name: "testName"
+// value: "testVal"
+// >
+// value: 42
+// timestamp: <
+// seconds: -62135596800
+// >
+// >
 // >
 // >
 }

prometheus/histogram.go

@@ -573,6 +573,7 @@ type constHistogram struct {
 sum float64
 buckets map[float64]uint64
 labelPairs []*dto.LabelPair
+exemplars []*dto.Exemplar
 }
 
 func (h *constHistogram) Desc() *Desc {
@@ -585,7 +586,6 @@ func (h *constHistogram) Write(out *dto.Metric) error {
 
 his.SampleCount = proto.Uint64(h.count)
 his.SampleSum = proto.Float64(h.sum)
-
 for upperBound, count := range h.buckets {
 buckets = append(buckets, &dto.Bucket{
 CumulativeCount: proto.Uint64(count),
@@ -596,6 +596,13 @@ func (h *constHistogram) Write(out *dto.Metric) error {
 if len(buckets) > 0 {
 sort.Sort(buckSort(buckets))
 }
+
+if len(h.exemplars) > 0 {
+for i := 0; i < len(buckets); i++ {
+buckets[i].Exemplar = h.exemplars[i]
+}
+}
+
 his.Bucket = buckets
 
 out.Histogram = his
@@ -604,6 +611,13 @@ func (h *constHistogram) Write(out *dto.Metric) error {
 return nil
 }
 
+func (h *constHistogram) GetExemplars() []*dto.Exemplar {
+if h != nil {
+return h.exemplars
+}
+return nil
+}
+
 // NewConstHistogram returns a metric representing a Prometheus histogram with
 // fixed values for the count, sum, and bucket counts. As those parameters
 // cannot be changed, the returned value does not implement the Histogram
@@ -617,6 +631,7 @@ func (h *constHistogram) Write(out *dto.Metric) error {
 //
 // NewConstHistogram returns an error if the length of labelValues is not
 // consistent with the variable labels in Desc or if Desc is invalid.
+
 func NewConstHistogram(
 desc *Desc,
 count uint64,
@@ -655,6 +670,48 @@ func MustNewConstHistogram(
 return m
 }
 
+func NewConstHistogramWithExemplar(
+desc *Desc,
+count uint64,
+sum float64,
+buckets map[float64]uint64,
+exemplars []*dto.Exemplar,
+labelValues ...string,
+
+) (Metric, error) {
+if desc.err != nil {
+return nil, desc.err
+}
+if err := validateLabelValues(labelValues, len(desc.variableLabels)); err != nil {
+return nil, err
+}
+return &constHistogram{
+desc: desc,
+count: count,
+sum: sum,
+buckets: buckets,
+exemplars: exemplars,
+labelPairs: MakeLabelPairs(desc, labelValues),
+}, nil
+}
+
+// MustNewConstHistogram is a version of NewConstHistogram that panics where
+// NewConstHistogram would have returned an error.
+func MustNewConstHistogramWithExemplar(
+desc *Desc,
+count uint64,
+sum float64,
+buckets map[float64]uint64,
+exemplars []*dto.Exemplar,
+labelValues ...string,
+) Metric {
+m, err := NewConstHistogramWithExemplar(desc, count, sum, buckets, exemplars, labelValues...)
+if err != nil {
+panic(err)
+}
+return m
+}
+
 type buckSort []*dto.Bucket
 
 func (s buckSort) Len() int {