Allow creating constant histogram metrics with a created timestamp

Closes #1535

Signed-off-by: Steven Swartz <sswartz@ripple.com>

prometheus/examples_test.go

@@ -456,6 +456,34 @@ func ExampleNewConstHistogram() {
 	// {"label":[{"name":"code","value":"200"},{"name":"method","value":"get"},{"name":"owner","value":"example"}],"histogram":{"sampleCount":"4711","sampleSum":403.34,"bucket":[{"cumulativeCount":"121","upperBound":25},{"cumulativeCount":"2403","upperBound":50},{"cumulativeCount":"3221","upperBound":100},{"cumulativeCount":"4233","upperBound":200}]}}
 }
 
+func ExampleNewConstHistogramWithCreatedTimestamp() {
+	desc := prometheus.NewDesc(
+		"http_request_duration_seconds",
+		"A histogram of the HTTP request durations.",
+		[]string{"code", "method"},
+		prometheus.Labels{"owner": "example"},
+	)
+
+	ct := time.Unix(0, 0).UTC()
+	h := prometheus.MustNewConstHistogramWithCreatedTimestamp(
+		desc,
+		4711, 403.34,
+		map[float64]uint64{25: 121, 50: 2403, 100: 3221, 200: 4233},
+		ct,
+		"200", "get",
+	)
+
+	// Just for demonstration, let's check the state of the histogram by
+	// (ab)using its Write method (which is usually only used by Prometheus
+	// internally).
+	metric := &dto.Metric{}
+	h.Write(metric)
+	fmt.Println(toNormalizedJSON(metric))
+
+	// Output:
+	// {"label":[{"name":"code","value":"200"},{"name":"method","value":"get"},{"name":"owner","value":"example"}],"histogram":{"sampleCount":"4711","sampleSum":403.34,"bucket":[{"cumulativeCount":"121","upperBound":25},{"cumulativeCount":"2403","upperBound":50},{"cumulativeCount":"3221","upperBound":100},{"cumulativeCount":"4233","upperBound":200}],"createdTimestamp":"1970-01-01T00:00:00Z"}}
+}
+
 func ExampleNewConstHistogram_WithExemplar() {
 	desc := prometheus.NewDesc(
 		"http_request_duration_seconds",

prometheus/histogram.go

@@ -1336,6 +1336,48 @@ func MustNewConstHistogram(
 	return m
 }
 
+// NewConstHistogramWithCreatedTimestamp does the same thing as NewConstHistogram but sets the created timestamp
+func NewConstHistogramWithCreatedTimestamp(
+	desc *Desc,
+	count uint64,
+	sum float64,
+	buckets map[float64]uint64,
+	ct time.Time,
+	labelValues ...string,
+) (Metric, error) {
+	if desc.err != nil {
+		return nil, desc.err
+	}
+	if err := validateLabelValues(labelValues, len(desc.variableLabels.names)); err != nil {
+		return nil, err
+	}
+	return &constHistogram{
+		desc:       desc,
+		count:      count,
+		sum:        sum,
+		buckets:    buckets,
+		labelPairs: MakeLabelPairs(desc, labelValues),
+		createdTs:  timestamppb.New(ct),
+	}, nil
+}
+
+// MustNewConstHistogramWithCreatedTimestamp is a version of NewConstHistogramWithCreatedTimestamp that panics where
+// NewConstHistogramWithCreatedTimestamp would have returned an error.
+func MustNewConstHistogramWithCreatedTimestamp(
+	desc *Desc,
+	count uint64,
+	sum float64,
+	buckets map[float64]uint64,
+	ct time.Time,
+	labelValues ...string,
+) Metric {
+	m, err := NewConstHistogramWithCreatedTimestamp(desc, count, sum, buckets, ct, labelValues...)
+	if err != nil {
+		panic(err)
+	}
+	return m
+}
+
 type buckSort []*dto.Bucket
 
 func (s buckSort) Len() int {