Add information on leadership changes to oban.peer.election event

[notslang]

This is an attempt at including whether Oban leadership changed in the oban.peer.election

I added the key was_leader, which indicates if the current node was the leader before the election happened.

If was_leader=false and leader=true in a telemetry event, then we know that the current node just became the leader. If was_leader=true and leader=false then we know that the current node lost leadership.

Marking this as a draft because I haven't tested it out yet in my application. I think I can add some tests to test/oban/peers/postgres_test.exs and test/oban/peers/global_test.exs too. That seems to be where this telemetry event gets tested.

Also, lmk if you'd prefer a different name.

[sorentwo]

Marking this as a draft because I haven't tested it out yet in my application. I think I can add some tests to test/oban/peers/postgres_test.exs and test/oban/peers/global_test.exs too. That seems to be where this telemetry event gets tested.

That's the right place.

Also, lmk if you'd prefer a different name.

The was_leader name works for me. Most of our other predicates have a trailing question mark, so perhaps was_leader? instead?

[sorentwo]

@notslang Do you have time to add the tests or would you like us to pick up this change?

[notslang]

I think this is good to review now. Renamed was_leader -> was_leader? and tested it out in my application. Looking for events where leader=true and was_leader?=false seems to reliably detect nodes becoming leaders.

[sorentwo]

Excellent, thanks!