Optinally print OM created lines

Signed-off-by: Arthur Silva Sens <arthur.sens@coralogix.com>

examples/createdtimestamps/main.go

@@ -0,0 +1,60 @@
+// Copyright 2022 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.
+
+// A simple example of how to exposed created timestamps in OpenMetrics format.
+
+package main
+
+import (
+	"log"
+	"net/http"
+	"time"
+
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promhttp"
+)
+
+func main() {
+	requestDurations := prometheus.NewHistogram(prometheus.HistogramOpts{
+		Name:    "http_request_duration_seconds",
+		Help:    "A histogram of the HTTP request durations in seconds.",
+		Buckets: prometheus.ExponentialBuckets(0.1, 1.5, 5),
+	})
+
+	// Create non-global registry.
+	registry := prometheus.NewRegistry()
+	registry.MustRegister(
+		requestDurations,
+	)
+
+	go func() {
+		for {
+			// Record fictional latency.
+			now := time.Now()
+			requestDurations.Observe(time.Since(now).Seconds())
+			time.Sleep(600 * time.Millisecond)
+		}
+	}()
+
+	// Expose /metrics HTTP endpoint using the created custom registry.
+	http.Handle(
+		"/metrics", promhttp.HandlerFor(
+			registry,
+			promhttp.HandlerOpts{
+				EnableOpenMetrics:               true,
+				EnableOpenMetricsCreatedMetrics: true,
+			}),
+	)
+	// To test: curl -H 'Accept: application/openmetrics-text' localhost:8080/metrics
+	log.Fatalln(http.ListenAndServe(":8080", nil))
+}

prometheus/promhttp/http.go

@@ -180,7 +180,12 @@ func HandlerForTransactional(reg prometheus.TransactionalGatherer, opts HandlerO
 			w = gz
 		}
 
-		enc := expfmt.NewEncoder(w, contentType)
+		var enc expfmt.Encoder
+		if opts.EnableOpenMetricsCreatedMetrics {
+			enc = expfmt.NewEncoder(w, contentType, expfmt.WithCreatedLines())
+		} else {
+			enc = expfmt.NewEncoder(w, contentType)
+		}
 
 		// handleError handles the error according to opts.ErrorHandling
 		// and returns true if we have to abort after the handling.
@@ -371,6 +376,21 @@ type HandlerOpts struct {
 	// (which changes the identity of the resulting series on the Prometheus
 	// server).
 	EnableOpenMetrics bool
+	// If 'EnableOpenMetrics' is true, 'EnableOpenMetricsCreatedMetrics' allows
+	// to add extra '_created' lines for counters, histograms and summaries,
+	// as defined in the OpenMetrics specification (see
+	// https://github.com/OpenObservability/OpenMetrics/blob/main/specification/OpenMetrics.md#counter-1).
+	// Created timestamps are used to improve the accuracy of reset detection,
+	// but be aware that it also increases the size of the payload.
+	//
+	// Prometheus introduced the feature flag 'created-timestamp-zero-ingestion'
+	// in version 2.50.0, but with support limited to the Prometheus protobuf
+	// format. Starting in Prometheus XXXX, the feature flag will be extended
+	// to the OpenMetrics text format. If using Prometheus XXXX or later, it
+	// is recommended to enable the feature flag in Prometheus, otherwise enabling
+	// _created lines will result in increased cardinality and no improvements
+	// in reset detection.
+	EnableOpenMetricsCreatedMetrics bool
 	// ProcessStartTime allows setting process start timevalue that will be exposed
 	// with "Process-Start-Time-Unix" response header along with the metrics
 	// payload. This allow callers to have efficient transformations to cumulative