feat: Add observability for heartbeat checks (#38918)

This is intended to help operators respond to situations where a failing
heartbeat check due to an unhealthy backend service causes the entire edxapp
cluster to be ejected from the load balancer.

Also remove a debugging print line from a unit test.

Cherry picked from commits 3956db35ea52bc13eefffc5d1e76ef9aa233527f
and 4fc67f64c71a10c7ba94bf761414b299dfda762b (2U fork).