Part of #1664, implements #1799.
## Changes
- `ate.worker.state` now has `idle`, `partial`, `at_capacity` and
`unschedulable`. `assigned` is removed.
- ateapi picks the state by comparing allocated actor slots with
capacity, the same check the scheduler makes.
- Draining workers and workers with no reported capacity are
`unschedulable`. This wins over occupancy, so the sum over the states is
still the pool size.
- Every known pool reports all four states, set to 0 when empty.
- Updated the registry, `docs/observability.md` and the
autoscaled-workerpool demo (`assigned` -> `at_capacity`).
## Open questions
- Are we fine with `unschedulable` as a fourth state? cc. @JeffLuoo
Every new worker starts with capacity 0, so `at_capacity` would make an
HPA on it scale up on its own scale-up.
- This breaks queries on `ate_worker_state="assigned"`. Should be still
fine.
- The demo HPA is only right while each worker holds one actor. Pool
utilization needs slot counts, which needs a new instrument (#1664).
- [x] Tests pass
- [x] Appropriate changes to documentation are included in the PR
Krisztian F
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