@Override
    public void connectionClosed(SessionProtocol protocol, InetSocketAddress remoteAddr,
                                 InetSocketAddress localAddr, AttributeMap attrs) throws Exception {
        closed.incrementAndGet();
    }
A {@link ConnectionPoolListener} to count the number of connections which have been open and closed.
core/src/test/java/com/linecorp/armeria/client/CountingConnectionPoolListener.java