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

def add_item_metadata(self, handle, key, value):
        """
        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
        """
        self._storage_broker.add_item_metadata(handle, key, value)