Generate an event if more than X relays join

We should decide on what this threshold is.

If more than the threshold of relays join, generate an EVENT status update and send to all channels subscribed to the EVENT type.

added component::archived/metrics bot owner::metrics-team parent::23937 points::0.2 priority::low severity::normal status::assigned type::enhancement labels

The opposite of this, detecting relays leaving, is in #24073.

I am not immediately working on this ticket. Reassigning to metrics-team to indicate this.

Trac:
Owner: irl to metrics-team
Status: new to assigned

Trac:
Points: N/A to 0.2

changed time estimate to 1h 36m

mentioned in issue #24073

mentioned in issue #28276 (moved)

mentioned in issue tpo/network-health/team#148