This metric is what `moodle_users_active` should have been in the first
place. It uses Moodle's activity tracking instead of timestamps in the
user table, and as such might just show 0 on some instances. It uses
15mins as a cutoff for what is considered "active" and "now".
`moodle_users_active` remains untouched, even though it's badly named
(should be `_loggedin_today` or something...) and shows some discrepancy
to Moodle's activity tracking.