Add more metrics to Discourse Prometheus

I would love to see some of these user activity metrics exposed in the Prometheus exporter plugin for Discourse as well!

Simple counters for total posts, visits, searches, users, topics and a gauge for currently active users should be enough to do fun stuff like very granular graphs for what hours your site is busy, alerting if there is a sudden spike in new users, posts and so on, and it would be easier to correlate the existing performance stats with user activity.

The built-in graphs are great of course, but the more stuff I can add to a Grafana dashboard the happier I get :wink:

Happy to add more metrics here, we just need to be careful not to add too much load regularly.