Only pass a metrics fd to the daemon child if a metrics address was specified

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/28581

daemon.go

@@ -205,7 +205,9 @@ func daemonize(config appConfig, uid, gid uint) {
 	daemonUpdateFds(cmd, config.ListenHTTP)
 	daemonUpdateFds(cmd, config.ListenHTTPS)
 	daemonUpdateFds(cmd, config.ListenProxy)
-	config.ListenMetrics = daemonUpdateFd(cmd, config.ListenMetrics)
+	if config.ListenMetrics != 0 {
+		config.ListenMetrics = daemonUpdateFd(cmd, config.ListenMetrics)
+	}
 	// Start the process
 	if err = cmd.Start(); err != nil {