Gathers episode metrics from PolicyEvaluator instances.

def collect_metrics(local_evaluator=None,
                    remote_evaluators=[],
                    timeout_seconds=180):
    """Gathers episode metrics from PolicyEvaluator instances."""

    episodes, num_dropped = collect_episodes(
        local_evaluator, remote_evaluators, timeout_seconds=timeout_seconds)
    metrics = summarize_episodes(episodes, episodes, num_dropped)
    return metrics