fix(nextjs): Let flush finish in API routes

[lobsterkatie]

As discussed in more detail in the penultimate paragraph of this PR, when deployed to vercel, API route lambdas have been shutting down too soon for sentry events to get reliably sent to our servers (in spite of the use of flush). This fixes that by wrapping the response's .end() method (which is what triggers the lambda shutdown) such that it waits for flush to finish before emitting its finished signal.

Logs from API routes now consistently look like this:

[GET] /api/hello

Sentry Logger [Log]: [Tracing] Starting http.server transaction - GET /api/hello
Sentry Logger [Log]: [Tracing] Finishing http.server transaction - GET /api/hello
Sentry Logger [Log]: Flushing events...
Sentry Logger [Log]: Done flushing events

Fixes #3806
Fixes #3643
Fixes #3691
Fixes #3748
Fixes #3636

[github-actions]

size-limit report

Path	Size
@sentry/browser - CDN Bundle (gzipped)	21.46 KB (-0.01% 🔽)
@sentry/browser - Webpack	22.47 KB (0%)
@sentry/react - Webpack	22.5 KB (0%)
@sentry/browser + @sentry/tracing - CDN Bundle (gzipped)	28.97 KB (0%)

[kamilogorek]

I went through the broken integration tests. They are all failing because of request.url (absolute instead of relative, but you have a note about it in addRequestDataToEvent). We can update tests if that's desired behavior as well.
Linter and unit tests are just overlooked attributes on hub/scope.

[lobsterkatie]

I went through the broken integration tests.

Thanks!

They are all failing because of request.url (absolute instead of relative, but you have a note about it in addRequestDataToEvent). We can update tests if that's desired behavior as well.

Yup, I know that's the underlying problem. But even once I fixed the tests, one is still failing, for mysterious reasons. ¯\(ツ)/¯ I'm sure I'll figure it out. That and the linting stuff I just ran out of time to fix before I turned into a pumpkin last night.

[rhcarvalho]

Had a partial look because @AbhiPrasad asked :)
I'd beg to avoid adding more API surface to general packages.

I understand scope bleed, but that's a much larger topic, I wouldn't try to mix that and the flush fix in one PR.

[lobsterkatie]

Recording the results of an IRL convo with @rhcarvalho, @AbhiPrasad, and @kamilogorek.

The scope issue is sufficiently complicated (and wide-reaching) that it will be pushed off to another PR. Here, we'll dial it back to just keeping track of the open transaction.

[AbhiPrasad]

Nice!

[maccman]

I don't quite understand this - Vercel/Next.js supports return promises.

Because of this monkey patching I'm getting:
API resolved without sending a response for /api/meetings?start_at=2021-09-25T04%3A00%3A00.000Z&end_at=2021-09-26T03%3A59%3A59.999Z, this may result in stalled requests.