docs: public void signIn(String key, Collection<WebSocketConnection> connections) {
          if (connections.isEmpty()) {
              return;
          }
          Map<WebSocketConnection, WebSocketConnection> newMap = new HashMap<>();
          for (WebSocketConnection conn : connections) {
              newMap.put(conn, conn);
          }
          ConcurrentMap<WebSocketConnection, WebSocketConnection> bag = ensureConnectionList(key);
          bag.putAll(newMap);
      }