feat: metrics capabilities (#8318)

Add metrics capabilities to Codex. The `README.md` is up to date.

This will not be merged with the metrics before this PR of course:
https://github.com/openai/codex/pull/8350

codex-rs/Cargo.lock

@@ -1610,10 +1610,12 @@ dependencies = [
  "opentelemetry-otlp",
  "opentelemetry-semantic-conventions",
  "opentelemetry_sdk",
+ "pretty_assertions",
  "reqwest",
  "serde",
  "serde_json",
  "strum_macros 0.27.2",
+ "thiserror 2.0.17",
  "tokio",
  "tracing",
  "tracing-opentelemetry",