code: def save_metadata_json(self, filename: str, structure: JsonExportable) -> None:
          
          if self.compress_json:
              filename += '.json.xz'
          else:
              filename += '.json'
          save_structure_to_file(structure, filename)
          if isinstance(structure, (Post, StoryItem)):
              # log 'json ' message when saving Post or StoryItem
              self.context.log('json', end=' ', flush=True)
docs: Saves metadata JSON file of a structure.