Add Prometheus stats for admin count (#26284)

Add Prometheus stats for admin count (#26284)

* Add Prometheus stats for admin count

Fixes #20828

Reports time series for admin count. Counts are separated by state (active, AFK, or deadminned) and admin rank.

* Use static constructor instead of static readonly for the metric

Docs recommend this due to inconsistent execution of C# static constructors.

* Remove static usage, use IoC IMeterFactory.

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Co-authored-by: metalgearsloth <comedian_vs_clown@hotmail.com>