ref(browser-tests): Add waitForMetricRequest helper

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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

Test plan

• Ran element timing tests 5x with --repeat-each=5 — all 10 runs pass consistently

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[linear-code]

JS-2011