Feature: Add granular GitHub Action usage metrics to telemetry

[cocosheng-g]

Description

This issue tracks the addition of new telemetry metadata keys to support granular usage metrics for the Gemini CLI when running in GitHub Actions.

Specifically, this adds:

• GEMINI_CLI_GH_EVENT_NAME (172): The GitHub event name (e.g., issues, pull_request, schedule).
• GEMINI_CLI_GH_EVENT_NUMBER (173): The raw integer event number (Issue or PR ID). This avoids PII fingerprinting while relying on the repository context to provide global uniqueness.

These additions allow for tracking unique Repositories, Issues, and PRs touched by the CLI, and distinguish between different workflow triggers (Automated vs. Scheduled).

Related

• Tracking issue: https://github.com/google-gemini/maintainers-gemini-cli/issues/1482
• PR: feat(telemetry): add specific PR, issue, and custom tracking IDs for GitHub Actions #21129