Clarify what metrics are included in Prometheus

[erosb]

Feedback:

Regarding the Prometheus Exporter (separate issue this time). We say in our docs that all metrics will be exported to Prometheus with the hz_ prefix. The metrics are also available via the member JMX API. However, what I'm seeing is that only some metrics are exposed, both in 4.x and 5.x. If you view the member's JMX API with jconsole, Mission Control, etc., you'll see that only the "Metrics" object seems to be exported via Prometheus. This folder does not include HazelcastInstance beans, IMap beans, and more.
Question: What is the Prometheus Exporter supposed to be exporting?

slack thread: https://hazelcast.slack.com/archives/C9S7BV74Z/p1643322745720039

The question is how do we explain what metrics are available instead of "The metrics are also available via the member JMX API". Pointing to MetricDescriptorConstants.java as a reference point? :)

[JakeSCahill]

I suppose that would be better than not providing any guidance. I'll create a PR for this.