code: @Validate
      public void start() {
          metrics.register("executors", new MetricSet() {
              @Override
              public Map<String, Metric> getMetrics() {
                  return ImmutableMap.<String, Metric>of(
                          "executors.count", new Gauge<Integer>() {
                              @Override
                              public Integer getValue() {
                                  return getExecutors().length;
                              }
                          },
                          "schedulers.count", new Gauge<Integer>() {
  
                              @Override
                              public Integer getValue() {
                                  return getSchedulers().length;
                              }
                          },
                          "hung.count", new Gauge<Integer>() {
  
                              @Override
                              public Integer getValue() {
                                  return getHungTasks();
                              }
                          },
                          "completed.count", new Gauge<Integer>() {
  
                              @Override
                              public Integer getValue() {
                                  return getCompletedTasks();
                              }
                          }
                  );
              }
          });
  
          for (ManagedExecutorService service : executors) {
              metrics.register(service.name(), metricsForExecutor(service));
          }
  
          for (ManagedExecutorService service : schedulers) {
              metrics.register(service.name(), metricsForExecutor(service));
          }
      }
docs: Starts the extension. It registers the different metrics into the metric registry.