<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _aggregate_metrics(self, session_group):
 """Sets the metrics of the group based on aggregation_type."""


    if (self._request.aggregation_type == api_pb2.AGGREGATION_AVG or
        self._request.aggregation_type == api_pb2.AGGREGATION_UNSET):
      _set_avg_session_metrics(session_group)
    elif self._request.aggregation_type == api_pb2.AGGREGATION_MEDIAN:
      _set_median_session_metrics(session_group,
                                  self._request.aggregation_metric)
    elif self._request.aggregation_type == api_pb2.AGGREGATION_MIN:
      _set_extremum_session_metrics(session_group,
                                    self._request.aggregation_metric,
                                    min)
    elif self._request.aggregation_type == api_pb2.AGGREGATION_MAX:
      _set_extremum_session_metrics(session_group,
                                    self._request.aggregation_metric,
                                    max)
    else:
      raise error.HParamsError('Unknown aggregation_type in request: %s' %
                               self._request.aggregation_type)