refactor(watcher): simplify config file watcher initialization using sync.Once

internal/watcher/config_file_watcher.go

@@ -8,18 +8,17 @@ import (
 )
 
 var (
-	configDirWatcher   *DirWatcher
-	configDirWatcherMu sync.Mutex
+	configDirWatcher         *DirWatcher
+	configDirWatcherInitOnce sync.Once
 )
 
+func initConfigDirWatcher() {
+	t := task.RootTask("config_dir_watcher", false)
+	configDirWatcher = NewDirectoryWatcher(t, common.ConfigBasePath)
+}
+
 // create a new file watcher for file under ConfigBasePath.
 func NewConfigFileWatcher(filename string) Watcher {
-	configDirWatcherMu.Lock()
-	defer configDirWatcherMu.Unlock()
-
-	if configDirWatcher == nil {
-		t := task.RootTask("config_dir_watcher", false)
-		configDirWatcher = NewDirectoryWatcher(t, common.ConfigBasePath)
-	}
+	configDirWatcherInitOnce.Do(initConfigDirWatcher)
 	return configDirWatcher.Add(filename)
 }