DaemonCli: Move check into startMetricsServer

Fix TODO: move into startMetricsServer()
Fix errors.Wrap return nil when passed err is nil

Co-Authored-By: Sebastiaan van Stijn <thaJeztah@users.noreply.github.com>
Signed-off-by: HuanHuan Ye <logindaveye@gmail.com>

cmd/dockerd/daemon.go

@@ -208,14 +208,10 @@ func (cli *DaemonCli) start(opts *daemonOptions) (err error) {
 		return errors.Wrap(err, "failed to validate authorization plugin")
 	}
 
-	// TODO: move into startMetricsServer()
-	if cli.Config.MetricsAddress != "" {
-		if !d.HasExperimental() {
-			return errors.Wrap(err, "metrics-addr is only supported when experimental is enabled")
-		}
-		if err := startMetricsServer(cli.Config.MetricsAddress); err != nil {
-			return err
-		}
+	cli.d = d
+
+	if err := cli.startMetricsServer(cli.Config.MetricsAddress); err != nil {
+		return err
 	}
 
 	c, err := createAndStartCluster(cli, d)
@@ -230,8 +226,6 @@ func (cli *DaemonCli) start(opts *daemonOptions) (err error) {
 
 	logrus.Info("Daemon has completed initialization")
 
-	cli.d = d
-
 	routerOptions, err := newRouterOptions(cli.Config, d)
 	if err != nil {
 		return err

cmd/dockerd/metrics.go

@@ -5,10 +5,19 @@ import (
 	"net/http"
 
 	"github.com/docker/go-metrics"
+	"github.com/pkg/errors"
 	"github.com/sirupsen/logrus"
 )
 
-func startMetricsServer(addr string) error {
+func (cli *DaemonCli) startMetricsServer(addr string) error {
+	if addr == "" {
+		return nil
+	}
+
+	if !cli.d.HasExperimental() {
+		return errors.New("metrics-addr is only supported when experimental is enabled")
+	}
+
 	if err := allocateDaemonPort(addr); err != nil {
 		return err
 	}