code: public int count(String key) {
          ConcurrentMap<WebSocketConnection, WebSocketConnection> bag = registry.get(key);
          return null == bag ? 0 : bag.size();
      }
docs: Returns the connection count by key specified in this registry
  
  Note it might count connections that are closed but not removed from registry yet
  
  @param key
  the key
  @return connection count by key