docs: 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()