ephemeral statuses lost

[PietroPasotti]

Bug Description

at the moment status management in traefik is decentralized and little is done to prevent statuses from being overridden even within the same event.

To Reproduce

suppose the charm is in blocked because some relation sent bad data:

    charm.traefik_route.on.ready.emit(charm.model.get_relation("traefik-route"))
    with pytest.raises(StaticConfigMergeConflictError):
        charm.traefik.generate_static_config(_raise=True)

    # THEN the charm status is blocked
    assert isinstance(charm.unit.status, ops.BlockedStatus)

as soon as we receive another, unrelated event, the blocked status is lost.

    charm.on.update_status.emit()
    assert isinstance(charm.unit.status, ops.ActiveStatus)

Environment

n/a

Relevant log output

n/a

Additional context

Solution:
adopt collect-unit-status common exit hook pattern, and some sort of compound-status to store error statuses across hook contexts