Add prober to alert when there are unapplied changes and task workers have 0 tasks

[bjester]

Background

We have probers that run in an isolated environment and detect certain scenarios so that we can be alerted when the scenarios occur. Some probers use internal API endpoints to detect these scenarios.

Desired behavior

• An internal API endpoint should exist that returns the quantity of tasks actively being processed by task workers, and the quantity of unapplied changes in the database
• A prober should raise an exception when the quantity of changes is non-zero and the quantity of tasks actively being processed by task workers is zero

Value add

• Helps to alert us if our task processing of changes isn't working properly

[bjester]

Addressed in #3855