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/healthinfo_nonlinux.go

@@ -30,7 +30,7 @@ import (
 func getLocalDiskHwInfo(ctx context.Context, r *http.Request) madmin.ServerDiskHwInfo {
 	addr := r.Host
 	if globalIsDistErasure {
-		addr = GetLocalPeer(globalEndpoints)
+		addr = globalLocalNodeName
 	}
 
 	return madmin.ServerDiskHwInfo{
@@ -42,7 +42,7 @@ func getLocalDiskHwInfo(ctx context.Context, r *http.Request) madmin.ServerDiskH
 func getLocalOsInfo(ctx context.Context, r *http.Request) madmin.ServerOsInfo {
 	addr := r.Host
 	if globalIsDistErasure {
-		addr = GetLocalPeer(globalEndpoints)
+		addr = globalLocalNodeName
 	}
 
 	return madmin.ServerOsInfo{