enforce that composite monitors use links

[grosser]

No description provided.

[zdrve]

Observations (not necessarily problems):

1. It's very much a non-backwards-compatible change (mainly of interest to anyone using Kennel who isn't Zendesk)
2. If, as someone using kennel outside of Zendesk, you wanted to turn off this behaviour that this PR adds – i.e. you're fine with link-by-datadog-id, without the need to opt out each time – how would you do that?
3. The only way to opt out is to opt out of all validations for the monitor.
4. How is the mutable overridable array meant to be used? Can you sketch what that would look like for Zendesk's case?

[grosser]

1: it requires some code changes, but not too bad I'd hope
2: override the method in the monitor class to be noop
3: it's a setting so link validation can be turned off on it's own
4: ALLOWED_UNLINKED = YAML.load_file('our_allow_list.yam') or hard-coded list in the code

[zdrve]

Prefer a boolean setting:

Suggested change

	validate_using_links(data)
	validate_using_links(data) if require_using_links

Having a "setting" where the two values are "method with logic" or "override with a no-op block" is ... a weird way of programming.

[zdrve]

Particularly when the method/block in question effectively returns void

[grosser]

yeah it's not very smooth ... some more general skip_validations: [:foo] might be nicer ... but that's more refactoring and can be it's own PR

[grosser]

another option would be

eval "->(*) { v }", nil, file, line.to_i

to use validate_using_links: false
but that ends up making any setting callable which could end up ugly too

[zdrve]

If (when) we extend this validation to other linking types - e.g. monitor widgets in dashboards – what will ALLOWED_UNLINKED look like then?

[grosser]

will need to move up into Kennel::ALLOWED_UNLINKED but the structure can stay the same