code: public synchronized ProbeImpl getProbe(Class<?> probedClass, String key) {
          Map<String, ProbeImpl> classProbes = probesByKey.get(probedClass);
          if (classProbes != null) {
              return classProbes.get(key);
          }
          return null;
      }
docs: Get an existing instance of a probe with the specified source class and
  key.
  
  @param probedClass the probe source class
  @param key the probe key
  
  @return an existing probe or null