code: func (s *IdentityService) GetSignupTokens() ([]services.SignupToken, error) {
  	startKey := backend.Key(userTokensPrefix)
  	result, err := s.GetRange(context.TODO(), startKey, backend.RangeEnd(startKey), backend.NoLimit)
  	if err != nil {
  		return nil, trace.Wrap(err)
  	}
  	tokens := make([]services.SignupToken, len(result.Items))
  	for i, item := range result.Items {
  		var signupToken services.SignupToken
  		err = json.Unmarshal(item.Value, &signupToken)
  		if err != nil {
  			return nil, trace.Wrap(err)
  		}
  		tokens[i] = signupToken
  	}
  	return tokens, nil
  }
docs: // GetSignupTokens returns all non-expired user tokens