test&doc: add example and update doc for function package

function/watcher.go

@@ -3,12 +3,18 @@ package function
 import "time"
 
 // Watcher is used for record code excution time
+// Play: Todo
 type Watcher struct {
 	startTime int64
 	stopTime  int64
 	excuting  bool
 }
 
+// Start the watch timer.
+func NewWatcher() *Watcher {
+	return &Watcher{}
+}
+
 // Start the watch timer.
 func (w *Watcher) Start() {
 	w.startTime = time.Now().UnixNano()