code: func (m InboundAppIDAuthMethod) Authenticate(c context.Context, r *http.Request) (*auth.User, error) {
  	appID := r.Header.Get("X-Appengine-Inbound-Appid")
  	if appID == "" {
  		return nil, nil
  	}
  	id, err := identity.MakeIdentity("service:" + appID)
  	if err != nil {
  		return nil, err
  	}
  	return &auth.User{Identity: id}, nil
  }
docs: // Authenticate extracts peer's identity from the incoming request.