code: @Override public void onMetadataChange(JobExecutionState state, String key, Object oldValue, Object newValue) {
      if (_log.isPresent()) {
        _log.get().info("JobExection metadata change for " + state.getJobSpec().toShortString() +
                        key + ": '" + oldValue + "' --> '" + newValue + "'");
      }
    }
docs: {@inheritDoc}