Grafana dashboards: needs an update for 4.x because rabbitmq_process_max_tcp_sockets is no longer available

[garry-t]

Describe the bug

I use Grafana dashboard for Rabbitmq provided by your team. After ugrade from 3.13.x to 4.0.x I fount that

Reproduction steps

1. Prometheus + Grafana need to be installed
2. Import official dashboard ID 10991
3. Install Rabbitmq 3.13.x
4. Point attention to panel TCP sockets available
5. Upgrade Rabbitmq to 4.0.x
6. Check same panel.

Or just shorter flow

1. Prometheus + Grafana need to be installed
2. Import official dashboard ID 10991
3. Install rabbitmq 4.0.x
4. Point attention to panel TCP sockets available

Expected behavior

Metrics are available or dashboard needs to be updated.

Additional context

n/a

[michaelklishin]

@garry-t you are welcome to update the dashboard.

In 4.0, as the change log suggests, several I/O-related metrics were removed. They should be monitored outside of RabbitMQ.