ref(browser-tests): Add waitForMetricRequest helper

[github-actions]

Note

The pull request "ref(browser-tests): Add waitForMetricRequest helper" was created by @logaretm but did not reference an issue. Therefore this issue was created for better visibility in external tools like Linear.

Summary

• Adds a shared waitForMetricRequest helper to browser integration test utils, following the same page.waitForRequest pattern as waitForErrorRequest, waitForTransactionRequest, etc.
• Refactors element timing tests to use waitForMetricRequest instead of a custom createMetricCollector with polling-based waitForIdentifiers
• The new helper accumulates SerializedMetric[] across envelope requests and resolves when the callback returns true for the full collected set

[linear-code]

JS-2012