docs: def _parse_credentials(username, password, database, options):
      """Parse authentication credentials."""
      mechanism = options.get('authmechanism', 'DEFAULT')
      if username is None and mechanism != 'MONGODB-X509':
          return None
      source = options.get('authsource', database or 'admin')
      return _build_credentials_tuple(
          mechanism, source, username, password, options)