code: func createService(c context.Context, opts auth.Options) (*compute.Service, error) {
  	http, err := auth.NewAuthenticator(c, auth.OptionalLogin, opts).Client()
  	if err != nil {
  		return nil, err
  	}
  	service, err := compute.New(http)
  	if err != nil {
  		return nil, err
  	}
  	return service, nil
  }
docs: // createService returns a new Compute Engine API service.