[codex] Instrument rollout persistence bytes (#29498)

- Add 1%-sampled rollout persistence metrics that report per-item and
per-thread JSON byte totals before and after filtering when metrics
export is enabled.
- Tag each item with its exact response or event variant, including
nested turn-item kinds for conditionally persisted completion events, so
aggregate cloud-storage impact can be estimated by policy choice.

codex-rs/Cargo.lock

@@ -3979,6 +3979,7 @@ dependencies = [
  "chrono",
  "codex-git-utils",
  "codex-install-context",
+ "codex-otel",
  "codex-protocol",
  "codex-rollout",
  "codex-state",