code: func (m *Mercury) GetOne(ctx context.Context, key string, opts ...clientv3.OpOption) (*mvccpb.KeyValue, error) {
  	resp, err := m.Get(ctx, key, opts...)
  	if err != nil {
  		return nil, err
  	}
  
  	if resp.Count != 1 {
  		return nil, types.NewDetailedErr(types.ErrBadCount, fmt.Sprintf("key: %s", key))
  	}
  
  	return resp.Kvs[0], nil
  }
docs: // GetOne get one result or noting