expose pending tasks of eventloops as metrics

[warmuuh]

expose pending tasks of eventloops as metrics both for server and client.
Currently, some metrics are provided (reactor.netty.http.[client|server]). it would be nice to also see pending tasks as this is an important metric about the health of the system.

see https://gitter.im/reactor/reactor-netty?at=5fc0ec2466dcfa77e2a78389 for related question

Motivation

a current solution seems to be possible like this

 httpServer.doOnBind(config -> {
        config.loopResources()
            .onServer(httpServer.configuration().isPreferNative())
            .forEach(eventExecutor -> {
              if (eventExecutor instanceof SingleThreadEventExecutor) {
                var singleThreadEventExecutor = (SingleThreadEventExecutor) eventExecutor;
                Gauge.builder(SERVER_PENDING_TASK_METRIC, singleThreadEventExecutor::pendingTasks)
                    .description("Pending Tasks")
                    .tag(SERVER_THREAD_NAME, singleThreadEventExecutor.threadProperties().name())
                    .tag(SERVER_THREAD_STATE, singleThreadEventExecutor.threadProperties().state().name())
                    .register(registry);
              }
            });

Desired solution

httpServer::metric and TcpClient::metric should result in this metric to also be exposed