make monitor start() and stop() async

[spali]

⚠️⚠️⚠️ Since we do not accept all types of pull requests and do not want to waste your time. Please be sure that you have read pull request rules:
https://github.com/louislam/uptime-kuma/blob/master/CONTRIBUTING.md#can-i-create-a-pull-request-for-uptime-kuma

Tick the checkbox if you understand [x]:

• I have read and understand the pull request rules.

Description

• follow up to remember prometheus instance and expose it #2664

• Preperation for Add tags as labels to prometheus metrics #898 (Extend existing Prometheus labels to include tags #680).

• make monitor start() and stop() async to allow for i.e. db calls.

Type of change

Please delete any options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• Other

Checklist

• My code follows the style guidelines of this project
• I ran ESLint and other linters for modified files
• I have performed a self-review of my own code and tested it
• I have commented my code, particularly in hard-to-understand areas
  (including JSDoc for methods)
• My changes generate no new warnings
• My code needed automated testing. I have added them (this is optional task)

Screenshots (if any)

Please do not use any external image service. Instead, just paste in or drag and drop the image here, and it will be uploaded automatically.

[chakflying]

Tested rebased on master seems to work fine. Should be no reason blocking this to merge? @louislam

[CommanderStorm]

@louislam FYI: I put this in v2.1 as I don't see any blocking change and think this can be merged as is from my side. If you disgree, please remove it ^^

[CommanderStorm]

Thanks for the change.
All changes in this PR are small and uncontroversial (I agree with @spali on this one that refactoring the logic to be less blocking should be done in another PR)
⇒ merging with junior maintainer approval.