Add metrics for `aibridged` and `aibridgeproxyd`'s provider statuses. AI providers can be modified, and possibly misconfigured, at runtime. These metrics help operators understand the state of these provider definitions in case unexpected behaviour is observed.
## Description
Adds Prometheus metrics to the AI Bridge Proxy for observability into
proxy traffic and performance.
## Changes
* Add Metrics struct with the following metrics:
* `connect_sessions_total`: counts CONNECT sessions by type
(mitm/tunneled)
* `mitm_requests_total`: counts MITM requests by provider
* `inflight_mitm_requests`: gauge tracking in-flight requests by
provider
* `mitm_request_duration_seconds`: histogram of request latencies by
provider
* `mitm_responses_total`: counts responses by status code class
(2XX/3XX/4XX/5XX) and provider
* Register metrics with `coder_aibridgeproxyd_` prefix in CLI
* Unregister metrics on server close to prevent registry leaks
* Add `tunneledMiddleware` to track non-allowlisted CONNECT sessions
* Add tests for metric recording in both MITM and tunneled paths
Closes: https://github.com/coder/internal/issues/1185