code: func signFromSource(source TokenSource, req *http.Request) error {
  	token, err := source.Token()
  	if err != nil {
  		return err
  	}
  	if !token.Valid() {
  		return fmt.Errorf("token expired or invalid")
  	}
  	token.SetAuthHeader(req)
  	return nil
  }
docs: // signFromSource generates a token using the given source and uses it to sign the request.