docs: def update_metadata(self):
          """
          Update the metadata available for packages in repo.continuum.io.
  
          Returns a download worker.
          """
          if self._data_directory is None:
              raise Exception('Need to call `api.set_data_directory` first.')
  
          metadata_url = 'https://repo.continuum.io/pkgs/metadata.json'
          filepath = os.sep.join([self._data_directory, 'metadata.json'])
          worker = self.download_requests(metadata_url, filepath)
          return worker