feat(browser): Add onRequestSpanStart hook to browser tracing integration

[lforst]

This PR adds a hook to the browser tracing integration that allows to hook into the creation of request spans (fetch & xhr) and set attributes based on headers (can be extended with additional info in the future).

Primarily I was gonna use this to set an attribute/op on prefetch client requests in Next.js to show them as http.client.prefetch.

[github-actions]

size-limit report 📦

Path	Size	% Change	Change
@sentry/browser	23.26 KB	+0.25%	+58 B 🔺
@sentry/browser - with treeshaking flags	23.09 KB	+0.28%	+66 B 🔺
@sentry/browser (incl. Tracing)	36.91 KB	+0.23%	+86 B 🔺
@sentry/browser (incl. Tracing, Replay)	74.11 KB	+0.17%	+123 B 🔺
@sentry/browser (incl. Tracing, Replay) - with treeshaking flags	67.48 KB	+0.15%	+101 B 🔺
@sentry/browser (incl. Tracing, Replay with Canvas)	78.77 KB	+0.15%	+114 B 🔺
@sentry/browser (incl. Tracing, Replay, Feedback)	90.58 KB	+0.11%	+100 B 🔺
@sentry/browser (incl. Feedback)	39.65 KB	+0.16%	+63 B 🔺
@sentry/browser (incl. sendFeedback)	27.87 KB	+0.21%	+58 B 🔺
@sentry/browser (incl. FeedbackAsync)	32.65 KB	+0.17%	+54 B 🔺
@sentry/react	25.06 KB	+0.22%	+55 B 🔺
@sentry/react (incl. Tracing)	38.83 KB	+0.23%	+90 B 🔺
@sentry/vue	27.47 KB	+0.23%	+62 B 🔺
@sentry/vue (incl. Tracing)	38.64 KB	+0.24%	+94 B 🔺
@sentry/svelte	23.29 KB	+0.25%	+58 B 🔺
CDN Bundle	24.5 KB	+0.27%	+66 B 🔺
CDN Bundle (incl. Tracing)	36.94 KB	+0.26%	+96 B 🔺
CDN Bundle (incl. Tracing, Replay)	71.99 KB	+0.17%	+120 B 🔺
CDN Bundle (incl. Tracing, Replay, Feedback)	77.15 KB	+0.12%	+87 B 🔺
CDN Bundle - uncompressed	71.45 KB	+0.3%	+217 B 🔺
CDN Bundle (incl. Tracing) - uncompressed	109.21 KB	+0.25%	+271 B 🔺
CDN Bundle (incl. Tracing, Replay) - uncompressed	220.5 KB	+0.13%	+271 B 🔺
CDN Bundle (incl. Tracing, Replay, Feedback) - uncompressed	233.03 KB	+0.12%	+271 B 🔺
@sentry/nextjs (client)	40.3 KB	+0.23%	+94 B 🔺
@sentry/sveltekit (client)	37.35 KB	+0.26%	+97 B 🔺
@sentry/node	142.92 KB	-	-
@sentry/node - without tracing	96.13 KB	-0.01%	-1 B 🔽
@sentry/aws-serverless	120.47 KB	-	-

View base workflow run

[Lms24]

The hook sounds reasonable to me! Can we add an integration test for it?

[lforst]

The hook sounds reasonable to me! Can we add an integration test for it?

yes tests are missing yet - hence pr in draft mode

[Lms24]

Thx for adding tests and the updated description! Sounds reasonable to me to ship this with #15980 in mind.