Return and cache the list of metrics to collect.

def _get_metrics_to_collect(self, instance_key, additional_metrics):
        """
        Return and cache the list of metrics to collect.
        """
        if instance_key not in self.metrics_to_collect_by_instance:
            self.metrics_to_collect_by_instance[instance_key] = self._build_metric_list_to_collect(additional_metrics)
        return self.metrics_to_collect_by_instance[instance_key]