code: def _parse_session_run_index(self, event):
      
      metadata_string = event.log_message.message
      try:
        metadata = json.loads(metadata_string)
      except ValueError as e:
        logger.error(
            "Could not decode metadata string '%s' for step value: %s",
            metadata_string, e)
        return constants.SENTINEL_FOR_UNDETERMINED_STEP
  
      try:
        return metadata["session_run_index"]
      except KeyError:
        logger.error(
            "The session_run_index is missing from the metadata: %s",
            metadata_string)
        return constants.SENTINEL_FOR_UNDETERMINED_STEP
docs: Parses the session_run_index value from the event proto.
  
      Args:
        event: The event with metadata that contains the session_run_index.
  
      Returns:
        The int session_run_index value. Or
        constants.SENTINEL_FOR_UNDETERMINED_STEP if it could not be determined.