docs: public int getRegisteredResourceRequestCount(K key) {
          if(requestQueueMap.containsKey(key)) {
              Queue<AsyncResourceRequest<V>> requestQueue = getRequestQueueForExistingKey(key);
              // FYI: .size() is not constant time in the next call. ;)
              if(requestQueue != null) {
                  return requestQueue.size();
              }
          }
          return 0;
      }