docs: def add_item_metadata(self, handle, key, value):
          """Store the given key:value pair for the item associated with handle.
  
          :param handle: handle for accessing an item before the dataset is
                         frozen
          :param key: metadata key
          :param value: metadata value
          """
          _mkdir_if_missing(self._metadata_fragments_abspath)
  
          prefix = self._handle_to_fragment_absprefixpath(handle)
          fpath = prefix + '.{}.json'.format(key)
  
          _put_obj(fpath, value)