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/ext/skills/src/extension_tests.rs

@@ -15,6 +15,10 @@ struct RecordingMetrics {
 }
 
 impl ExtensionMetrics for RecordingMetrics {
+    fn counter(&self, name: &str, _inc: i64, _tags: &[(&str, &str)]) {
+        panic!("unexpected counter: {name}");
+    }
+
     fn histogram(&self, name: &str, value: i64, _tags: &[(&str, &str)]) {
         self.samples
             .lock()

codex-rs/ext/skills/src/render_observability_tests.rs

@@ -18,6 +18,10 @@ struct RecordingMetrics {
 }
 
 impl ExtensionMetrics for RecordingMetrics {
+    fn counter(&self, name: &str, _inc: i64, _tags: &[(&str, &str)]) {
+        panic!("unexpected counter: {name}");
+    }
+
     fn histogram(&self, name: &str, value: i64, tags: &[(&str, &str)]) {
         self.samples
             .lock()

codex-rs/ext/skills/tests/skills_extension.rs

@@ -2294,6 +2294,10 @@ impl RecordingMetrics {
 }
 
 impl ExtensionMetrics for RecordingMetrics {
+    fn counter(&self, name: &str, _inc: i64, _tags: &[(&str, &str)]) {
+        panic!("unexpected counter: {name}");
+    }
+
     fn histogram(&self, name: &str, value: i64, tags: &[(&str, &str)]) {
         self.samples
             .lock()