code: def _get_collections(self, db_names=None):
          
          if db_names:
              db_list = db_names
          else:
              db_list = self.client.list_database_names()
  
          for db_name in db_list:
              db = self.client.get_database(db_name)
              for col_name in db.list_collection_names():
                  size = db[col_name].g
                  yield f"{db_name}.{col_name}"
docs: Internal function to return all the collections for every database.
          include a list of db_names to filter the list of collections.