docs: def _DeleteClientStats(self,
                           limit,
                           retention_time,
                           cursor=None):
      """Deletes up to `limit` ClientStats older than `retention_time`."""
      cursor.execute(
          "DELETE FROM client_stats WHERE timestamp < FROM_UNIXTIME(%s) LIMIT %s",
          [mysql_utils.RDFDatetimeToTimestamp(retention_time), limit])
      return cursor.rowcount