daemon control / jsonclient: fix connection leak due to open request body

Also:
- Defensive measures in control http server (1s timeouts)
(prevent the leak, even if request body is not closed)
- Add prometheus metrics to track control socket latencies
(were used for debugging)

client/jsonclient.go

@@ -30,7 +30,10 @@ func jsonRequestResponse(c http.Client, endpoint string, req interface{}, res in
 	resp, err := c.Post("http://unix"+endpoint, "application/json", &buf)
 	if err != nil {
 		return err
-	} else if resp.StatusCode != http.StatusOK {
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
 		var msg bytes.Buffer
 		io.CopyN(&msg, resp.Body, 4096)
 		return errors.Errorf("%s", msg.String())