rc: fix default value of --metrics-addr

Before this fix it was empty string, which isn't a good default for a
stringArray.

fs/rc/rc.go

@@ -85,7 +85,7 @@ var OptionsInfo = fs.Options{{
 	Groups:  "RC",
 }, {
 	Name:    "metrics_addr",
-	Default: []string{""},
+	Default: []string{},
 	Help:    "IPaddress:Port or :Port to bind metrics server to",
 	Groups:  "Metrics",
 }}.

fs/rc/rcserver/metrics.go

@@ -37,7 +37,7 @@ func init() {
 // If the server wasn't configured the *Server returned may be nil
 func MetricsStart(ctx context.Context, opt *rc.Options) (*MetricsServer, error) {
 	jobs.SetOpt(opt) // set the defaults for jobs
-	if opt.MetricsHTTP.ListenAddr[0] != "" {
+	if len(opt.MetricsHTTP.ListenAddr) > 0 {
 		// Serve on the DefaultServeMux so can have global registrations appear
 		s, err := newMetricsServer(ctx, opt)
 		if err != nil {