Usage tracking for Slack Integration

In support of creating a MAU number for ~"group::ecosystem" , we need to track "active" users of each of our integrations. As noted in the discussion in #225678 (closed), there's no easy way to do this in aggregate, so we'll need to track usage for each service and aggregate from there. This issue is specifically tackling our integration with Slack.

Activity to track

Slack "user activity" counts as:

• Any time a user takes an action that triggers the webhook that sends activity data to the connected Slack workspace

/cc @lvanc @mnohr