code: func defaultSigner(c context.Context) (*signer, error) {
  	s := auth.GetSigner(c)
  	if s == nil {
  		return nil, errors.Reason("a default signer is not available").Err()
  	}
  	info, err := s.ServiceInfo(c)
  	if err != nil {
  		return nil, errors.Annotate(err, "failed to grab the signer info").Err()
  	}
  	return &signer{
  		Email:     info.ServiceAccountName,
  		SignBytes: s.SignBytes,
  	}, nil
  }
docs: // defaultSigner uses the default app account for signing.