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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 _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