code: func (c *EtcdGroupService) GroupCountMembers(ctx context.Context, groupName string) (int, error) {
  	ctxT, cancel := context.WithTimeout(ctx, transactionTimeout)
  	defer cancel()
  	etcdRes, err := clientInstance.Get(ctxT, memberKey(groupName, ""), clientv3.WithPrefix(), clientv3.WithCountOnly())
  	if err != nil {
  		return 0, err
  	}
  	return int(etcdRes.Count), nil
  }
docs: // GroupCountMembers get current member amount in group