docs: def _generate_overlays(self):
          """Return dictionary of overlays generated from added item metadata."""
          overlays = defaultdict(dict)
          for handle in self._storage_broker.iter_item_handles():
              identifier = dtoolcore.utils.generate_identifier(handle)
              item_metadata = self._storage_broker.get_item_metadata(handle)
              for k, v in item_metadata.items():
                  overlays[k][identifier] = v
  
          return overlays