Add service state history

[tonyandrewmeyer]

It would be convenient to have a history of the service state, similar to the status history, so that you can do things like check for “running → restart → running”

• Harness cannot do this either at the moment, so this would be adding another advantage to Scenario.
• Status histories rightly belong to the context rather than the state, so this would as well.
• Presumably, we’d want to offer some ability to control what happens in a replan or restart (and maybe other Pebble operations?) rather than just assuming that restart means “add a service state of inactive then add another one of running”.

Rough example of possible usage:

import ops
import scenario

class MyCharm(ops.CharmBase):
    def __init__(self, framework):
        super().__init__(framework)
        framework.observe(self.on.foo_pebble_ready, self._on_pebble)

    def _on_pebble(self, event):
        ...
        self.unit.get_container("foo").restart("srv")
        ...

ctx = scenario.Context(MyCharm)
container = scenario.Container(service_statuses={
    "srv": ops.pebble.ServiceStatus.ACTIVE,
})
state_in = scenario.State(containers={"foo": container})
state_out = ctx.run(ctx.on.foo_pebble_ready, state_in)
assert ctx.service_history["foo"]["srv"] == [
    ops.pebble.ServiceStatus.ACTIVE,
    ops.pebble.ServiceStatus.INACTIVE,
]
assert state_out.containers["foo"].service_status["srv"] == ops.pebble.ServiceStatus.ACTIVE

[tonyandrewmeyer]

Hmm, this might actually be a dupe of #34.

[tonyandrewmeyer]

Hmm, this might actually be a dupe of #34.

I think it's close enough that this doesn't need to stay open.