code: func (r *MintOAuthTokenGrantRPC) checkRequestFormat(req *minter.MintOAuthTokenGrantRequest) error {
  	switch {
  	case req.ServiceAccount == "":
  		return fmt.Errorf("service_account is required")
  	case req.ValidityDuration < 0:
  		return fmt.Errorf("validity_duration must be positive, not %d", req.ValidityDuration)
  	case req.EndUser == "":
  		return fmt.Errorf("end_user is required")
  	}
  	if _, err := identity.MakeIdentity(req.EndUser); err != nil {
  		return fmt.Errorf("bad end_user - %s", err)
  	}
  	if err := utils.ValidateTags(req.AuditTags); err != nil {
  		return fmt.Errorf("bad audit_tags - %s", err)
  	}
  	return nil
  }
docs: // checkRequestFormat returns an error if the request is obviously wrong.