fix(browser): Ensure keepalive flag is correctly set for parallel requests

[mydea]

We noticed that sometimes request would remain in a seemingly pending state.
After some investigation, we found out that the limit of 64kb for keepalive-enabled fetch requests is not per request but for all parallel requests running at the same time.

This PR fixes this by keeping track of how large the pending body sizes are, and setting keepalive accordingly.

I tried this in my local reproduction app, where it now worked as expected!

When do we use keepalive now

We use keepalive if:

• total currently pending body size < 60k
• total # of pending requests < 15

This should defensively work for relevant cases.

Why do we want to have keepalive, when possible?

Especially for replays and transactions, having keepalive enabled means we can capture more data when a user leaves the page, which would otherwise be lost. This lead to problems with Next.js transaction collection, for example. In Replay, we want to be able to make sure to do a final flush before the user leaves the page.

Spec reference

• fetch spec

Fixes #7546
Fixes #6049

[AbhiPrasad]

Should we also update the doc string?

The spec says:

If the sum of contentLength and inflightKeepaliveBytes is greater than 64 kibibytes, then return a network error.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	20.44 KB (+0.12% 🔺)
@sentry/browser - ES5 CDN Bundle (minified)	63.55 KB (+0.08% 🔺)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	19.13 KB (+0.14% 🔺)
@sentry/browser - ES6 CDN Bundle (minified)	56.59 KB (+0.1% 🔺)
@sentry/browser - Webpack (gzipped + minified)	21.6 KB (+0.12% 🔺)
@sentry/browser - Webpack (minified)	71.67 KB (+0.08% 🔺)
@sentry/react - Webpack (gzipped + minified)	21.63 KB (+0.12% 🔺)
@sentry/nextjs Client - Webpack (gzipped + minified)	51.96 KB (+0.05% 🔺)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	27.77 KB (+0.09% 🔺)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	26.08 KB (+0.11% 🔺)
@sentry/replay ES6 CDN Bundle (gzipped + minified)	44.22 KB (+0.01% 🔺)
@sentry/replay - Webpack (gzipped + minified)	38.28 KB (0%)
@sentry/browser + @sentry/tracing + @sentry/replay - ES6 CDN Bundle (gzipped + minified)	62.4 KB (+0.04% 🔺)
@sentry/browser + @sentry/replay - ES6 CDN Bundle (gzipped + minified)	55.95 KB (+0.07% 🔺)

[mydea]

Updated the doc accordingly!

[HazAT]

ref: #1496