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

function/watcher_example_test.go

@@ -0,0 +1,22 @@
+package function
+
+import "fmt"
+
+func ExampleWatcher() {
+	w := NewWatcher()
+
+	w.Start()
+
+	longRunningTask()
+
+	w.Stop()
+
+	// eapsedTime := w.GetElapsedTime().Milliseconds()
+
+	fmt.Println("foo")
+
+	w.Reset()
+
+	// Output:
+	// foo
+}