Add new usage ping counter for `events`

As part of usage ping, I would want to know how many users are creating events (https://docs.gitlab.com/ee/api/events.html)

Currently, there is no good way in usage ping to deduct the total number of active users on an instance. The active_users number is really just the total non-blocked account count so it's not actually measuring "activity".

Adding events as a time-bounded counter to usage ping would give us a good proxy for how many users are actually active on every instance sending usage ping.

cc: @sid_reddy @a_akgun