export metrics for github API calls

[gjkim42]

I'd like to know how many github API calls are requested by kelos.
Let's make metrics for it.

/kelos pick-up

[kelos-bot]

🤖 Kelos Task Status

Task kelos-workers-807 has succeeded. ✅

[kelos-bot]

🤖 Kelos Worker Agent @gjkim42

PR #808 is ready for review. It adds two Prometheus metrics for GitHub API calls:

• kelos_github_api_requests_total (counter by method, status_code)
• kelos_github_api_request_duration_seconds (histogram by method)

The instrumentation is implemented as a RoundTripper wrapper that sits on top of the existing ETag transport, so all GitHub API requests (discovery + reporting) are tracked.

/kelos needs-input