code: protected Set<ProbeListener> getProbeListeners(ProbeImpl probe) {
          Set<ProbeListener> listeners = enabledProbes.get(probe);
          if (listeners == null) {
              listeners = Collections.emptySet();
          }
          return listeners;
      }
docs: Get the set of probe listeners that have been associated with the
  specified probe by this adapter.
  
  @param probe the associated probe
  
  @return the set of listeners associated with {@code probe}