code: public void addListener(HealthCheckRegistryListener listener) {
          listeners.add(listener);
          for (Map.Entry<String, HealthCheck> entry : healthChecks.entrySet()) {
              listener.onHealthCheckAdded(entry.getKey(), entry.getValue());
          }
      }
docs: Adds a {@link HealthCheckRegistryListener} to a collection of listeners that will be notified on health check
  registration. Listeners will be notified in the order in which they are added. The listener will be notified of all
  existing health checks when it first registers.
  
  @param listener listener to add