feat(emqx_prometheus): expose live_connections stats to prometheus

[olcai]

This exposes the stats live_connections.count and live_connections.max to Prometheus.

Sister to #9930 for v5.

Fixes EMQX-8852.

PR Checklist

Please convert it to a draft if any of the following conditions are not met. Reviewers may skip over until all the items are checked:

• Added tests for the changes (no: there is no test coverage of the prometheus code)
• Changed lines covered in coverage report
• Change log has been added to changes/ dir
• appup files updated (execute scripts/update-appup.sh emqx)
• For internal contributor: there is a jira ticket to track this change
• If there should be document changes, a PR to emqx-docs.git is sent, or a jira ticket is created to follow up
• In case of non-backward compatible changes, reviewer should check this item as a write-off, and add details in Backward Compatibility section

[coveralls]

Pull Request Test Coverage Report for Build 4125685681

• 0 of 2 (0.0%) changed or added relevant lines in 1 file are covered.
• 31 unchanged lines in 6 files lost coverage.
• Overall coverage decreased (-0.05%) to 74.939%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
apps/emqx_prometheus/src/emqx_prometheus.erl	0	2	0.0%

Files with Coverage Reduction	New Missed Lines	%
apps/emqx_auth_mnesia/src/emqx_acl_mnesia_migrator.erl	1	94.37%
src/emqx_alarm_handler.erl	1	30.77%
src/emqx_broker.erl	3	83.23%
src/emqx_router_helper.erl	3	80.85%
src/emqx_cm.erl	8	81.63%
src/emqx_reason_codes.erl	15	88.57%

Totals
Change from base Build 4123916300:	-0.05%
Covered Lines:	14748
Relevant Lines:	19680

---

💛 - Coveralls

[zmstone]

oops chanelog conflict .