docs: def metadata_filter_as_string(metadata_filter):
      """Convert a filter, represented as a dictionary with 'additional' and 'excluded' entries, to a string"""
      if not isinstance(metadata_filter, dict):
          return metadata_filter
  
      additional = metadata_filter.get('additional', [])
      if additional == 'all':
          entries = ['all']
      else:
          entries = [key for key in additional if key not in _JUPYTEXT_CELL_METADATA]
  
      excluded = metadata_filter.get('excluded', [])
      if excluded == 'all':
          entries.append('-all')
      else:
          entries.extend(['-' + e for e in excluded])
  
      return ','.join(entries)