feat: add support for Pebble check events

[tonyandrewmeyer]

Adds support for the {container-name}-pebble-check-failed and {container-name}-pebble-check-recovered events that will be available in Juju 3.6.

Example usage:

ctx = Context(MyCharm)
check_info = CheckInfo("http-check", failures=7, status=ops.pebble.CheckStatus.DOWN)
container = Container("my-container", check_infos={check_info})
state = State(containers={container})

ctx.run(ctx.on.pebble_check_failed(container=container, info=check_info), state)

Also updates the support for Pebble custom notice events to align with the 7.x approach (event from ctx.on, notices are a frozenset).

Updated usage:

ctx = Context(MyCharm)
notices = [
    Notice(key="example.com/a", occurrences=10),
    Notice(key="example.com/b", last_data={"bar": "baz"}),
    Notice(key="example.com/c"),
]
container = Container("my-container", notices=notices)
state = State(containers={container})

ctx.run(ctx.on.pebble_custom_notice(container=container, notice=notices[-1]), state)

[tonyandrewmeyer]

I wonder if the defaults should be reversed here, to CheckStatus.DOWN and e.g. failures=3. The normal situation should be that the check is passing, but if it's passing then I doubt people would be creating the event and putting it into the container (if that was done automatically with a tool, then it would not require the defaults).

I think observing and therefore testing pebble-check-failed is probably more likely than pebble-check-recovered, so that would lean me towards making a failing check the one that requires the least work. Or maybe charms will always observe both, in a kind of do/undo pattern?

[benhoyt]

I think what you have (UP, 0) is better, as it matches the Pebble "defaults". So it seems more explicit and less surprising to me than having it (DOWN, 3) as a default, even if it's a bit more work for tests that use it.

[tonyandrewmeyer]

The merge broke the linting check because I forgot to reorder them. I'll fix that later. The test pass if you use ops main - I assume we would not merge this until that's become 2.15 anyway, so will trigger the tests before that.

[benhoyt]

I think what you have (UP, 0) is better, as it matches the Pebble "defaults". So it seems more explicit and less surprising to me than having it (DOWN, 3) as a default, even if it's a bit more work for tests that use it.

[benhoyt]

Looking good -- thanks. Just one minor comment about the CheckInfo argument name.