Add Guardian v2 approval review metrics (#39224)

## What changed

- Pass metrics bound to the active turn's model to approval-review contributors.
- Add counter support to `ExtensionMetrics` while preserving host session attribution.
- Record Guardian v2 tool-call score lag and count stale-score fallbacks with the `score_lag` reason.

## Testing

- Cover counter forwarding and session metadata tags.
- Verify Guardian v2 metric samples for current and stale scores.

GitOrigin-RevId: 0ec34736d29d2e37ed217a27e773b4f6230ae38a

codex-rs/core/src/session/mod.rs

@@ -204,7 +204,7 @@ use codex_protocol::exec_output::StreamOutput;
 mod code_mode_warning;
 pub(crate) mod context_window;
 mod environment;
-mod extension_metrics;
+pub(crate) mod extension_metrics;
 mod handlers;
 mod inject;
 mod input_queue;