Simplify access to local node name (#11907)

The local node name is heavily used in tracing, create a new global 
variable to store it. Multiple goroutines can access it since it won't be
changed later.

cmd/metrics.go

@@ -538,7 +538,7 @@ func storageMetricsPrometheus(ch chan<- prometheus.Metric) {
 	}
 
 	server := getLocalServerProperty(globalEndpoints, &http.Request{
-		Host: GetLocalPeer(globalEndpoints),
+		Host: globalLocalNodeName,
 	})
 
 	onlineDisks, offlineDisks := getOnlineOfflineDisksStats(server.Disks)