Apply code review feedback for OpenTelemetry support

- Export rate limit header constants in `github/github.go`.
- Add `HeaderRequestID` constant.
- Update `otel/transport.go` to use exported constants.
- Remove redundant comment in `example/otel/main.go`.
- Fix module configuration and formatting.
- Ensure `StatusTooManyRequests` logic in `CheckResponse` is preserved.
- Run script/generate.sh to ensure generated files are up to date.

Co-authored-by: merchantmoh-debug <241568449+merchantmoh-debug@users.noreply.github.com>

Author: google-labs-jules[bot]