change prometheus status in hook on state change

[thorinaboenke]

Task

#253

Description

Fixes engine_running metric, that kept reporting running after the engine loop thread crashed.
Every engine state change now goes through _set_state(), which calls an _on_state_change() hook, and Service overrides this method to update the metric (STOPPING keeps reporting running, so start()/stop() no longer set it themselves.

How Has This Been Tested?

new unit tests

Checklist

• This Pull-Request goes to the development branch.
• I have successfully run prek locally.
• I have added tests to cover my changes.
• I have linked the issue-id to the task-description.
• I have performed a self-review of my own code.

---

^{Need help on this PR? Tag @codesmith-bot with what you need. Autofix is disabled.}