Returns queue statistics (coroutine)

            :param tube_name:
                If specified, statistics by a specific tube is returned,
                else statistics about all tubes is returned

async def statistics(self, tube_name=None):
        """
            Returns queue statistics (coroutine)

            :param tube_name:
                If specified, statistics by a specific tube is returned,
                else statistics about all tubes is returned
        """
        args = None
        if tube_name is not None:
            args = (tube_name,)

        res = await self._conn.call('{}.statistics'.format(self._namespace), args)
        if self._conn.version < (1, 7):  # pragma: nocover
            return res.body[0][0]
        return res.body[0]