By @ayanda-D: make sure (non-replicated) CQs (classic queues) emit leader and members metrics, just like replicated QQs

[michaelklishin]

This is #12422 by @Ayanda-D with a different title and a slightly different explanation from me.

This change may seem controversial at first given the removal of classic queue mirroring starting with 4.0 but this is just a matter of unifying metric keys between non-replicated CQs and replicated QQs.

So with this change, CQs will report a leader and a list of members (of one node, just the leader) as stats keys. For v4.0.x, this seems perfectly fine.

In fact, this will help improve https://github.com/rabbitmq/rabbitmqadmin-ng with its stricter JSON deserializer (compared to the original Python script).

[michaelklishin]

I will have to rebase this to make check_workflow pass. Will do so later today.

[michaelklishin]

Submitted a review for #12422: the new test fails on Actions with

=== Location: [{classic_queue_SUITE,'-leader_locator_client_local/1-lc$^0/1-0-',[87](https://github.com/rabbitmq/rabbitmq-server/pull/classic_queue_suite.src.html#87)},
              {classic_queue_SUITE,'-leader_locator_client_local/1-lc$^0/1-0-',[91](https://github.com/rabbitmq/rabbitmq-server/pull/classic_queue_suite.src.html#91)},
              {test_server,ts_tc,1793},
              {test_server,run_test_case_eval1,1302},
              {test_server,run_test_case_eval,1234}]
=== === Reason: {assertEqual,
                     [{module,classic_queue_SUITE},
                      {line,87},
                      {expression,
                          "rabbit_ct_broker_helpers : rpc ( Config , Server , rabbit_amqqueue , info , [ Leader0 , [ leader , members ] ] )"},
                      {expected,
                          [{leader,'rmq-ct-cluster_size_3-2-21192@localhost'},
                           {members,
                               ['rmq-ct-cluster_size_3-2-21192@localhost']}]},
                      {value,[{leader,''},{members,''}]}]}
  in function  classic_queue_SUITE:'-leader_locator_client_local/1-lc$^0/1-0-'/2 (classic_queue_SUITE.erl, line 87)
  in call from classic_queue_SUITE:'-leader_locator_client_local/1-lc$^0/1-0-'/2 (classic_queue_SUITE.erl, line 91)
  in call from test_server:ts_tc/3 (test_server.erl, line 1793)
  in call from test_server:run_test_case_eval1/6 (test_server.erl, line 1302)
  in call from test_server:run_test_case_eval/9 (test_server.erl, line 1234)

This assertion generally does not belong to this test. It does not test info items returned by CQ processes.