fix: add checks and log_targets to ops.testing

[amandahla]

Add checks and log_targets to ops.testing add_layer so it is possible to test it via Harness.

Fixes #1112

[benhoyt]

Thanks for this. The structure is very good, but there are a few minor issues with the types and the merging logic.

[dimaqq]

[misunderstood smth, yanked comment]

[benhoyt]

Code looks good now, approving! Just a couple of nit issues in the tests. I'll also ask another Charm Tech team member to review.

[amandahla]

off topic: Why the PR title validation fails if I change it to "fix: add checks and log_targets to ops.testing"?

[benhoyt]

off topic: Why the PR title validation fails if I change it to "fix: add checks and log_targets to ops.testing"?

@amandahla Must have been a glitch -- I just changed it back to that now, and it ran fine. We'll keep an eye on it.

[benhoyt]

@amandahla We require two reviews for PRs on this repo, so I've asked one more Charm Tech reviewer to review this (probably in the next couple of days), but it's looking good!

[tonyandrewmeyer]

A couple of small suggestions, but looks great, thanks!

[tonyandrewmeyer]

This is asserting that the plan is equal to the second layer, right? Because override: replace was used.

I think the test would be clearer (and more resilient) if it made that explicit. Something like (untested):

Suggested change

	client.add_layer(
	'foo',
	pebble.Layer("""\
	log-targets:
	baz:
	override: replace
	type: loki
	location: https://example123.com:3100/loki/api/v1/push
	services:
	- foo
	labels:
	key1: value1
	"""),
	combine=True,
	)
	plan = client.get_plan()
	assert (
	textwrap.dedent("""\
	log-targets:
	baz:
	labels:
	key1: value1
	location: https://example123.com:3100/loki/api/v1/push
	override: replace
	services:
	- foo
	type: loki
	""")
	== plan.to_yaml()
	)
	layer = pebble.Layer("""\
	log-targets:
	baz:
	override: replace
	type: loki
	location: https://example123.com:3100/loki/api/v1/push
	services:
	- foo
	labels:
	key1: value1
	"""
	)
	client.add_layer(
	'foo',
	layer,
	combine=True,
	)
	plan = client.get_plan()
	assert layer.to_yaml() == plan.to_yaml()

[amandahla]

I dont have a strong opinion about it so I don't mind changing it but to me the existing code looks explicit too :)
I follow the pattern in the test_add_layer_combine_override_replace. I'm afraid that changing just one of them would look confusing.