Keep API request metrics out of Statsig exports

[copyberry]

Keep API request metrics out of Statsig exports

What changed

• Treat codex.api_request and codex.api_request.duration_ms as runtime-only metrics for Statsig exporters, matching the existing tool-call metric behavior.
• Keep both metrics available through other OTLP exporters.

Testing

• Extend the Statsig filtering test to cover both API request metrics.
• Verify the OTLP HTTP exporter still sends the API request count and duration.

[github-actions]

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