code: func (s *CommonStorageDB) GetPrivateDataMetadataByHash(namespace, collection string, keyHash []byte) ([]byte, error) {
  	if !s.metadataHint.metadataEverUsedFor(namespace) {
  		return nil, nil
  	}
  	vv, err := s.GetValueHash(namespace, collection, keyHash)
  	if err != nil || vv == nil {
  		return nil, err
  	}
  	return vv.Metadata, nil
  }
docs: // GetPrivateDataMetadataByHash implements corresponding function in interface DB. For additional details, see
  // decription of the similar function 'GetStateMetadata'