code: func CallerFuncHandler(h Handler) Handler {
  	return FuncHandler(func(r *Record) error {
  		r.Ctx = append(r.Ctx, "fn", formatCall("%+n", r.Call))
  		return h.Log(r)
  	})
  }
docs: // CallerFuncHandler returns a Handler that adds the calling function name to
  // the context with key "fn".