code: def add_item_metadata(self, handle, key, value):
          
          self._storage_broker.add_item_metadata(handle, key, value)
docs: Add metadata to a specific item in the :class:`dtoolcore.ProtoDataSet`.
  
          :param handle: handle representing the relative path of the item in the
                         :class:`dtoolcore.ProtoDataSet`
          :param key: metadata key
          :param value: metadata value