Returns
        -------
        List of Arctic library names from a cached collection (global per mongo cluster) in mongo.
        Long term list_libraries should have a use_cached argument.

def _list_libraries_cached(self, newer_than_secs=-1):
        """
        Returns
        -------
        List of Arctic library names from a cached collection (global per mongo cluster) in mongo.
        Long term list_libraries should have a use_cached argument.
        """
        _ = self._conn  # Ensures the connection exists and cache is initialized with it.
        cache_data = self._cache.get('list_libraries', newer_than_secs)
        if cache_data:
            logger.debug('Library names are in cache.')
            return cache_data

        return self._list_libraries()