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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _parse_session_run_index(self, event):
 """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. """

    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