BUILD-11444 Replace decision file gate with CI_METRICS_ENABLED env var

[julien-carsique-sonarsource]

Summary

Replaces the presence-only file gate in the WarpBuild job-completed.sh hook:

• Removes ${CI_METRICS_DIR}/enabled presence check.
• Reads $CI_METRICS_ENABLED directly — set by the pre-job hook via $GITHUB_ENV.

Why: eliminates the dual-source-of-truth (env var + file) and the stale-file risk on persistent runners.

Companion PRs

• github-runners-infra — PR #416: pre-job hook writes to $GITHUB_ENV; post-job hook reads env var; test updated.
• ci-ami-images — same gate replacement in WarpBuild job-started.sh
• gh-action_cache — drops "Resolve CI metrics gate" step; simplifies cache-metrics-prep if:

Test plan

• Deployed to dev runner and validated on sonar-dummy E2E workflow (before merging)

[hashicorp-vault-sonar-prod]

BUILD-11444

[gitar-bot]

Code Review ✅ Approved

Replaces the file-based metrics gate with the CI_METRICS_ENABLED environment variable to eliminate redundant state tracking. No issues found.

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues
0 New dependency risks

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[Copilot]

Pull request overview

This PR updates the CI metrics post-job hook (ci-metrics/job-completed.sh) to use a single feature-flag source of truth: the CI_METRICS_ENABLED environment variable (written via $GITHUB_ENV by the pre-job hook), replacing the previous presence-only gate file check under ${CI_METRICS_DIR}/enabled. This reduces the risk of stale gate files on persistent runners and removes the dual-source-of-truth behavior.

Changes:

• Remove the ${CI_METRICS_DIR}/enabled file presence gate.
• Gate metrics collection on CI_METRICS_ENABLED == "true" and improve skip logging with the observed value.

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