add idle time status

[Peddle]

What is this?

Adds idle time reporting to potassium status endpoint.

Why?

IdleTime is a useful metric for a load balancer to track to know when to scale down replicas.

How did you test it works without regressions?

unit tests passing + manual testing

If this is a new feature what may a critical error look like?

Things to consider to not repeat mistakes we've learned from many times

• [ ] If critical errors fire do we ping team in some obvious way (e.g., slack)?
• [ ] Are there debug logs + a way to see these logs if we need to debug?
• [ ] Is this documented enough a dev could work on this code without getting stuck or having to ping you?