## Description
This PR adds Prometheus metrics for aibridge's API-key failover, giving visibility into key pool health and failover behavior per provider.
The following metrics are introduced:
- **`key_pool_state`** (gauge): number of keys currently in each state (`valid`, `temporary`, `permanent`) per provider, sampled at scrape time.
- **`key_pool_state_transitions_total`** (counter): key state transitions during failover, labeled by `reason` (`rate_limited`, `unauthorized`, `forbidden`).
- **`key_pool_exhaustions_total`** (counter): times a pool ran out of usable keys, labeled by `outcome` (`rate_limited`, `auth_failed`).
- **`key_pool_failover_attempts`** (histogram): keys attempted before success or exhaustion (per interception for bridged requests, per request for passthrough).
## Changes
- Moves `MarkKeyOnStatus` and key-pool error handling onto `*keypool.Pool`.
- Attaches metrics to each provider's key pool at install time, on construction and on provider reload.
- Adds a scrape-time state collector and a `KeyPools()` accessor on the bridge pool to feed it.
- Tracks per-request key attempts in the bridged and passthrough failover paths.
- Adds test coverage for the new metrics across the keypool unit tests, the bridged intercept failover tests, and the passthrough failover test.
Closes https://github.com/coder/internal/issues/1447
Closes https://linear.app/codercom/issue/AIGOV-198/aibridge-key-failover-observability
> [!NOTE]
> Initially generated by Claude Opus 4.7, modified and reviewed by @ssncferreira
Susana Ferreira
01ec5e4577