code: def collections(self):
          
          iterator = self._firestore_api.list_collection_ids(
              self._database_string, metadata=self._rpc_metadata
          )
          iterator.client = self
          iterator.item_to_value = _item_to_collection_ref
          return iterator
docs: List top-level collections of the client's database.
  
          Returns:
              Sequence[~.firestore_v1beta1.collection.CollectionReference]:
                  iterator of subcollections of the current document.