Adds a `workload_create_duration_seconds` Prometheus histogram to the
supervisor, observed around the workload manager `create()` call:
- `backend` label: `kubernetes` | `compute` | `docker` — set once from
the configured workload manager
- `outcome` label: `success` | `error` — the per-outcome counts double
as a create error rate
Registered on the supervisor's existing metrics registry, so it's
exposed on the existing `/metrics` endpoint with no config changes.
Notes:
- Covers cold creates only; warm starts and restores return before
reaching `create()`.
- A create may include backend-internal retries, so one observation can
span multiple attempts.
- Fixed low cardinality: 2 active label sets per deployment × 10
buckets.
Saadi Myftija
8b405711ac