Export individual Playwright test durations to Prometheus

Checkly shows the duration of individual tests in Playwright Check Suite results. The Prometheus endpoint currently exports checkly_playwright_check_duration_seconds, which measures the entire check.

We use Playwright checks to monitor how long production user journeys take. The overall check duration is not usable for this purpose: it includes Checkly setup and runner overhead that users never experience. A change in that number does not tell us whether the product became slower. We need the duration of the Playwright test itself to track UX performance in Grafana.

Please export per-test duration as a Prometheus metric with the check ID and a stable test identifier.