MV3: Metrics / Sentry Test Cases

[hilvmason]

• Ensure pending segment events are not lost after service worker re-start
• Ensure Sentry continues to work after service worker re-start

[danjm]

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[gauthierpetetin]

This ticket may no longer be required as if all goes well, we won't have to re-start service worker.
Google said service workers will not have to be restarted but this needs to be confirmed.
We'll keep this ticket open until we have more info about service worker persistence.
In case there are edge cases, where service workers get disconnected, we'll have to refine this ticket more, and test Extension behaviour when we kill the service worker manually.

We'll have more info once our E2E tests work with MV3: #21496