code: func (t *TxStructure) HLen(key []byte) (int64, error) {
  	metaKey := t.encodeHashMetaKey(key)
  	meta, err := t.loadHashMeta(metaKey)
  	if err != nil {
  		return 0, errors.Trace(err)
  	}
  	return meta.FieldCount, nil
  }
docs: // HLen gets the number of fields in a hash.