Helper method to discover the metrics defined for a training job.

def _metric_names_for_training_job(self):
        """Helper method to discover the metrics defined for a training job.
        """
        training_description = self._sage_client.describe_training_job(TrainingJobName=self._training_job_name)

        metric_definitions = training_description['AlgorithmSpecification']['MetricDefinitions']
        metric_names = [md['Name'] for md in metric_definitions]

        return metric_names