Retrieve any associated metrics for the policy.

def get_metrics(self, start, end, period):
        """Retrieve any associated metrics for the policy."""
        values = {}
        default_dimensions = {
            'Policy': self.policy.name, 'ResType': self.policy.resource_type,
            'Scope': 'Policy'}

        metrics = list(self.POLICY_METRICS)

        # Support action, and filter custom metrics
        for el in itertools.chain(
                self.policy.resource_manager.actions,
                self.policy.resource_manager.filters):
            if el.metrics:
                metrics.extend(el.metrics)

        session = utils.local_session(self.policy.session_factory)
        client = session.client('cloudwatch')

        for m in metrics:
            if isinstance(m, six.string_types):
                dimensions = default_dimensions
            else:
                m, m_dimensions = m
                dimensions = dict(default_dimensions)
                dimensions.update(m_dimensions)
            results = client.get_metric_statistics(
                Namespace=DEFAULT_NAMESPACE,
                Dimensions=[
                    {'Name': k, 'Value': v} for k, v
                    in dimensions.items()],
                Statistics=['Sum', 'Average'],
                StartTime=start,
                EndTime=end,
                Period=period,
                MetricName=m)
            values[m] = results['Datapoints']
        return values