metrics/librato: use http package to replace http method names (ether…

…eum#26535)

metrics/librato/client.go

@@ -80,7 +80,7 @@ func (lc *LibratoClient) PostMetrics(batch Batch) (err error) {
 		return
 	}
 
-	if req, err = http.NewRequest("POST", MetricsPostUrl, bytes.NewBuffer(js)); err != nil {
+	if req, err = http.NewRequest(http.MethodPost, MetricsPostUrl, bytes.NewBuffer(js)); err != nil {
 		return
 	}