code: func (a *HistoricalApi) clusterMetrics(request *restful.Request, response *restful.Response) {
  	key := core.HistoricalKey{ObjectType: core.MetricSetTypeCluster}
  	a.processMetricRequest(key, request, response)
  }
docs: // clusterMetrics returns a metric timeseries for a metric of the Cluster entity.