push.go: push metrics to remote storage in gzip-compressed form

This should reduce the needed network bandwidth by ~10x for reading the metrics at remote storage side

push.go

@@ -9,6 +9,8 @@ import (
 	"net/http"
 	"net/url"
 	"time"
+
+	"compress/gzip"
 )
 
 // InitPushProcessMetrics sets up periodic push for 'process_*' metrics to the given pushURL with the given interval.
@@ -86,6 +88,9 @@ func (s *Set) InitPush(pushURL string, interval time.Duration, extraLabels strin
 //
 // It is OK calling InitPushExt multiple times with different pushURL -
 // in this case metrics are pushed to all the provided pushURL urls.
+//
+// It is OK calling InitPushExt multiple times with different writeMetrics -
+// in this case all the metrics generated by writeMetrics callbacks are writte to pushURL.
 func InitPushExt(pushURL string, interval time.Duration, extraLabels string, writeMetrics func(w io.Writer)) error {
 	if interval <= 0 {
 		return fmt.Errorf("interval must be positive; got %s", interval)
@@ -111,15 +116,30 @@ func InitPushExt(pushURL string, interval time.Duration, extraLabels string, wri
 		ticker := time.NewTicker(interval)
 		var bb bytes.Buffer
 		var tmpBuf []byte
+		zw := gzip.NewWriter(&bb)
 		for range ticker.C {
 			bb.Reset()
 			writeMetrics(&bb)
 			if len(extraLabels) > 0 {
 				tmpBuf = addExtraLabels(tmpBuf[:0], bb.Bytes(), extraLabels)
 				bb.Reset()
-				bb.Write(tmpBuf)
+				if _, err := bb.Write(tmpBuf); err != nil {
+					panic(fmt.Errorf("BUG: cannot write %d bytes to bytes.Buffer: %s", len(tmpBuf), err))
+				}
+			}
+			tmpBuf = append(tmpBuf[:0], bb.Bytes()...)
+			bb.Reset()
+			zw.Reset(&bb)
+			if _, err := zw.Write(tmpBuf); err != nil {
+				panic(fmt.Errorf("BUG: cannot write %d bytes to gzip writer: %s", len(tmpBuf), err))
+			}
+			req, err := http.NewRequest("GET", pushURL, &bb)
+			if err != nil {
+				log.Printf("ERROR: metrics.push: cannot initialize request for metrics push to %q: %s", pushURLRedacted, err)
 			}
-			resp, err := c.Post(pushURL, "text/plain", &bb)
+			req.Header.Set("Content-Type", "text/plain")
+			req.Header.Set("Content-Encoding", "gzip")
+			resp, err := c.Do(req)
 			if err != nil {
 				log.Printf("ERROR: metrics.push: cannot push metrics to %q: %s", pushURLRedacted, err)
 				continue