Fix CumulativeCount value of +Inf bucket created from exemplar

prometheus/histogram.go

@@ -613,7 +613,7 @@ func (h *constHistogram) Write(out *dto.Metric) error {
 // to send it to Prometheus in the Collect method.
 //
 // buckets is a map of upper bounds to cumulative counts, excluding the +Inf
-// bucket.
+// bucket. The +Inf bucket is implicit, and its value is equal to the provided count.
 //
 // NewConstHistogram returns an error if the length of labelValues is not
 // consistent with the variable labels in Desc or if Desc is invalid.

prometheus/metric.go

@@ -187,7 +187,7 @@ func (m *withExemplarsMetric) Write(pb *dto.Metric) error {
 } else {
 // The +Inf bucket should be explicitly added if there is an exemplar for it, similar to non-const histogram logic in https://github.com/prometheus/client_golang/blob/main/prometheus/histogram.go#L357-L365.
 b := &dto.Bucket{
-CumulativeCount: proto.Uint64(pb.Histogram.Bucket[len(pb.Histogram.GetBucket())-1].GetCumulativeCount()),
+CumulativeCount: proto.Uint64(pb.Histogram.GetSampleCount()),
 UpperBound: proto.Float64(math.Inf(1)),
 Exemplar: e,
 }

prometheus/metric_test.go

@@ -79,10 +79,14 @@ func TestWithExemplarsMetric(t *testing.T) {
 }
 }
 
-infBucket := metric.GetHistogram().Bucket[len(metric.GetHistogram().Bucket)-1].GetUpperBound()
+infBucket := metric.GetHistogram().Bucket[len(metric.GetHistogram().Bucket)-1]
 
-if infBucket != math.Inf(1) {
-t.Errorf("want %v, got %v", math.Inf(1), infBucket)
+if want, got := math.Inf(1), infBucket.GetUpperBound(); want != got {
+t.Errorf("want %v, got %v", want, got)
+}
+
+if want, got := uint64(4711), infBucket.GetCumulativeCount(); want != got {
+t.Errorf("want %v, got %v", want, got)
 }
 })
 }