ref(replay): Try to make flush on unload fully sync

[mydea]

This PR adjusts #6854 to be fully sync.

I tried it in a test app, and it seemed to send the request out - I tried setting up a proxy to properly see logs, and it appeared correct, although not 100% sure if that matches real-world usage.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	19.82 KB (+0.01% 🔺)
@sentry/browser - ES5 CDN Bundle (minified)	61.47 KB (0%)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	18.5 KB (+0.02% 🔺)
@sentry/browser - ES6 CDN Bundle (minified)	54.77 KB (0%)
@sentry/browser - Webpack (gzipped + minified)	20.22 KB (+0.05% 🔺)
@sentry/browser - Webpack (minified)	66.17 KB (-0.01% 🔽)
@sentry/react - Webpack (gzipped + minified)	20.24 KB (+0.01% 🔺)
@sentry/nextjs Client - Webpack (gzipped + minified)	47.51 KB (+0.07% 🔺)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	26.73 KB (-0.05% 🔽)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	25.02 KB (-0.02% 🔽)
@sentry/replay ES6 CDN Bundle (gzipped + minified)	43.3 KB (-0.75% 🔽)
@sentry/replay - Webpack (gzipped + minified)	38.65 KB (+0.59% 🔺)
@sentry/browser + @sentry/tracing + @sentry/replay - ES6 CDN Bundle (gzipped + minified)	60.52 KB (-0.6% 🔽)

[mydea]

@billyvg feel free to merge this yourself if this appears good to you!

[billyvg]

The network request still gets cancelled, although I don't think the transport is using keepalive -- how would I confirm if it's using it or not?

[Lms24]

I don't think the transport is using keepalive

It should be using fetch's keepalive flag:
https://github.com/getsentry/sentry-javascript/blob/223a20f3063bf8d501546132fdd2dca56c69c3d6/packages/browser/src/transports/fetch.ts#L31

[Lms24]

how would I confirm if it's using it or not

My best guess would be to start some requests that'd take a long time and perform a page navigation. This shouldn't cancel the requests.

[billyvg]

OK I added some logpoints to confirm when keepalive is true and it seems to work, although still getting some missing segments.

[mydea]

Ahhh, the problem is actually this: #6910

We just didn't even call the sync flush when unloading the page 😅 I was kind of under the impression we had this, but I guess not!