add process start time header to client_golang prometheus (#1278)

* add process start time header to client_golang prometheus

Signed-off-by: Han Kang <hankang@google.com>

* Apply suggestions from code review

Co-authored-by: Bartlomiej Plotka <bwplotka@gmail.com>
Signed-off-by: Han Kang <hankang@google.com>

* fix up changes due to incorporation of feedback

* fix lint issues

---------

Signed-off-by: Han Kang <hankang@google.com>
Co-authored-by: Bartlomiej Plotka <bwplotka@gmail.com>

prometheus/promhttp/http.go

@@ -37,6 +37,7 @@ import (
 	"fmt"
 	"io"
 	"net/http"
+	"strconv"
 	"strings"
 	"sync"
 	"time"
@@ -50,6 +51,7 @@ const (
 	contentTypeHeader      = "Content-Type"
 	contentEncodingHeader  = "Content-Encoding"
 	acceptEncodingHeader   = "Accept-Encoding"
+	processStartTimeHeader = "Process-Start-Time-Unix"
 )
 
 var gzipPool = sync.Pool{
@@ -121,6 +123,9 @@ func HandlerForTransactional(reg prometheus.TransactionalGatherer, opts HandlerO
 	}
 
 	h := http.HandlerFunc(func(rsp http.ResponseWriter, req *http.Request) {
+		if !opts.ProcessStartTime.IsZero() {
+			rsp.Header().Set(processStartTimeHeader, strconv.FormatInt(opts.ProcessStartTime.Unix(), 10))
+		}
 		if inFlightSem != nil {
 			select {
 			case inFlightSem <- struct{}{}: // All good, carry on.
@@ -366,6 +371,14 @@ type HandlerOpts struct {
 	// (which changes the identity of the resulting series on the Prometheus
 	// server).
 	EnableOpenMetrics 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
+	// counters (e.g. OpenTelemetry) or generally _created timestamp estimation per
+	// scrape target.
+	// NOTE: This feature is experimental and not covered by OpenMetrics or Prometheus
+	// exposition format.
+	ProcessStartTime time.Time
 }
 
 // gzipAccepted returns whether the client will accept gzip-encoded content.