code: func CallerFileHandler(h Handler) Handler {
  	return FuncHandler(func(r *Record) error {
  		r.Ctx = append(r.Ctx, "caller", fmt.Sprint(r.Call))
  		return h.Log(r)
  	})
  }
docs: // CallerFileHandler returns a Handler that adds the line number and file of
  // the calling function to the context with key "caller".