## Summary
This PR fixes OTLP HTTP trace export in runtimes where the previous
exporter setup was unreliable, especially around app-server usage. It
also removes the old `codex_otel::otel_provider` compatibility shim and
switches remaining call sites over to the crate-root
`codex_otel::OtelProvider` export.
## What changed
- Use a runtime-safe OTLP HTTP trace exporter path for Tokio runtimes.
- Add an async HTTP client path for trace export when we are already
inside a multi-thread Tokio runtime.
- Make provider shutdown flush traces before tearing down the tracer
provider.
- Add loopback coverage that verifies traces are actually sent to
`/v1/traces`:
- outside Tokio
- inside a multi-thread Tokio runtime
- inside a current-thread Tokio runtime
- Remove the `codex_otel::otel_provider` shim and update remaining
imports.
## Why
I hit cases where spans were being created correctly but never made it
to the collector. The issue turned out to be in exporter/runtime
behavior rather than the span plumbing itself. This PR narrows that gap
and gives us regression coverage for the actual export path.
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