Exclude some metrics from aggregated responses

[michaelklishin]

As of rabbitmq/rabbitmq-prometheus#24, we aggregate metrics by default. It drastically reduces the size of the response but also makes certain kinds of troubleshooting impossible. Per-object metrics can be enabled via config file.

Some specific metrics, however, represent the state of a single queue and make no sense or can be misleading when aggregated. Raft commit index of a quorum queue is one such example. We should consider excluding them from aggregation. Missing some metrics would suck but misleading data is equally bad if not worse.

@Vanlightly @gerhard

[gerhard]

Help is wanted & will be appreciated.