code: private MetricGroup up_down_host_(String host, boolean up) {
          final GroupName group = GroupName.valueOf(
                  getBasePath(),
                  Tags.valueOf(singletonMap("host", MetricValue.fromStrValue(host))));
          return new SimpleMetricGroup(group, singleton(up ? UP_METRIC : DOWN_METRIC));
      }
docs: Return a metric group indicating if the host is up or down.