code: func (m *GoogleOAuth2Method) GetUserCredentials(c context.Context, r *http.Request) (*oauth2.Token, error) {
  	accessToken, err := accessTokenFromHeader(r.Header.Get("Authorization"))
  	if err != nil {
  		return nil, err
  	}
  	return &oauth2.Token{
  		AccessToken: accessToken,
  		TokenType:   "Bearer",
  	}, nil
  }
docs: // GetUserCredentials implements UserCredentialsGetter.