Wire metrics client into memories extension (#24567)

## Summary

- let the memories extension capture the process-global OTEL metrics
client at install time
- keep app-server/TUI/exec extension construction APIs unchanged
- store the metrics client for future memory metrics without emitting
any metrics yet

## Test plan

- `just fmt`
- `just bazel-lock-update`
- `just bazel-lock-check`
- Not run: tests/clippy per request; CI will cover them

codex-rs/app-server/src/extensions.rs

@@ -18,7 +18,7 @@ where
 {
     let mut builder = ExtensionRegistryBuilder::<Config>::new();
     codex_guardian::install(&mut builder, guardian_agent_spawner);
-    codex_memories_extension::install(&mut builder);
+    codex_memories_extension::install(&mut builder, codex_otel::global());
     Arc::new(builder.build())
 }