code: void addListenerByProbe(ProbeImpl probe, ProbeListener listener) {
          Set<ProbeListener> listeners = listenersByProbe.get(probe);
          if (listeners == null) {
              listeners = new CopyOnWriteArraySet<ProbeListener>();
              listenersByProbe.putIfAbsent(probe, listeners);
              listeners = listenersByProbe.get(probe);
          }
          listeners.add(listener);
      }
docs: Add the specified listener to the collection of listeners associated
  with the specified probe.
  
  @param probe the probe that fires for the listener
  @param listener the listener that's driven by the probe