Prometheus: expose memory breakdown metrics

[michaelklishin]

Closes #11743.

How to test

1. Start a node with two plugins: rabbitmq_management,rabbitmq_prometheus
2. Run curl http://guest:guest@localhost:15692/metrics | grep "classic\|quorum\|stream"
3. Observe the values (if this is a blank node, many will be equal to zero)
4. Declare a few CQs, QQs, streams
5. Re-run curl
6. Observe the values
7. Delete all CQs
8. Re-run, observe the metrics
9. Delete all QQs
10. Re-run, observe the metrics
11. Run curl http://guest:guest@localhost:15692/metrics | grep "memory" and inspect the rest, verify help labels, etc