push.go: limit the maximum timeout for pushing metrics to the provided interval between pushes

This should guarantee that metrics are pushed regularly with the provided interval.
If the remote storage cannot keep up with push frequency, then timeout errors will be logged.

push.go

@@ -81,6 +81,9 @@ func initPush(pushURL string, interval time.Duration, extraLabels string, writeM
 	if err := validateTags(extraLabels); err != nil {
 		panic(fmt.Errorf("BUG: invalid extraLabels=%q: %s", extraLabels, err))
 	}
+	c := &http.Client{
+		Timeout: interval,
+	}
 	go func() {
 		ticker := time.NewTicker(interval)
 		var bb bytes.Buffer
@@ -93,7 +96,7 @@ func initPush(pushURL string, interval time.Duration, extraLabels string, writeM
 				bb.Reset()
 				bb.Write(tmpBuf)
 			}
-			resp, err := http.Post(pushURL, "text/plain", &bb)
+			resp, err := c.Post(pushURL, "text/plain", &bb)
 			if err != nil {
 				log.Printf("cannot push metrics to %q: %s", pushURL, err)
 				continue