code: public static void register(Monitor<?> monitor) {
      if (monitor instanceof SpectatorMonitor) {
        ((SpectatorMonitor) monitor).initializeSpectator(BasicTagList.EMPTY);
      } else if (!isEmptyComposite(monitor)) {
        ServoMeter m = new ServoMeter(monitor);
        PolledMeter.remove(registry, m.id());
        PolledMeter.monitorMeter(registry, m);
        monitorMonitonicValues(monitor);
      }
    }
docs: Register a custom monitor.