tracing: NumSubscribers() to use atomic instead of mutex (#11219)

globalSubscribers.NumSubscribers() is heavily used in S3 requests and it
uses mutex, use atomic.Load instead since it is faster

Co-authored-by: Anis Elleuch <anis@min.io>

cmd/notification.go

@@ -1375,7 +1375,7 @@ func sendEvent(args eventArgs) {
 		return
 	}
 
-	if globalHTTPListen.HasSubscribers() {
+	if globalHTTPListen.NumSubscribers() > 0 {
 		globalHTTPListen.Publish(args.ToEvent(false))
 	}