private void initSession(SessionProtocol desiredProtocol, SerializationFormat serializationFormat,
                             PoolKey poolKey, ChannelFuture connectFuture, Promise<Channel> sessionPromise) {
        assert connectFuture.isSuccess();

        final Channel ch = connectFuture.channel();
        final EventLoop eventLoop = ch.eventLoop();
        assert eventLoop.inEventLoop();

        ch.pipeline().addLast(
                new HttpSessionHandler(this, ch, sessionPromise, connectTimeoutMillis,
                                       desiredProtocol, serializationFormat, poolKey, clientFactory));
    }
Returns the number of open connections on this {@link HttpChannelPool}.
core/src/main/java/com/linecorp/armeria/client/HttpChannelPool.java