feat(nextjs): Trace pageloads in App Router

[lforst]

This PR adds a very basic form of App Router pageload tracing to the Next.js SDK. This will allow us to see server component spans inside the trace, alongside the pageload span.

This works by using the new experimental.clientTraceMetadata option added in Next.js 15 (14.3.0-canary.64) which will inject meta tags, containing OTEL trace propagation data. The SDK automatically sets this option when a supporting version is detected.

As a side-effect, this allows the SDK to associate errors thrown in server components (server side) with errors captured in error boundaries in the browser.

Example trace:

What doesn't work yet

This PR does not yet include the functionality to attach serverside spans to the request span of the pageload span. This requires some hack that we can explore in a follow-up PR: #12182

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser	21.74 KB (0%)
@sentry/browser (incl. Tracing)	32.88 KB (0%)
@sentry/browser (incl. Tracing, Replay)	68.25 KB (0%)
@sentry/browser (incl. Tracing, Replay) - with treeshaking flags	61.66 KB (0%)
@sentry/browser (incl. Tracing, Replay with Canvas)	72.28 KB (0%)
@sentry/browser (incl. Tracing, Replay, Feedback)	84.33 KB (0%)
@sentry/browser (incl. Feedback)	37.75 KB (0%)
@sentry/browser (incl. sendFeedback)	26.31 KB (0%)
@sentry/browser (incl. FeedbackAsync)	30.73 KB (0%)
@sentry/react	24.43 KB (0%)
@sentry/react (incl. Tracing)	35.88 KB (0%)
@sentry/vue	25.68 KB (0%)
@sentry/vue (incl. Tracing)	34.7 KB (0%)
@sentry/svelte	21.88 KB (0%)
CDN Bundle	24.28 KB (0%)
CDN Bundle (incl. Tracing)	34.35 KB (0%)
CDN Bundle (incl. Tracing, Replay)	68.04 KB (0%)
CDN Bundle (incl. Tracing, Replay, Feedback)	73.02 KB (0%)
CDN Bundle - uncompressed	71.46 KB (0%)
CDN Bundle (incl. Tracing) - uncompressed	101.87 KB (0%)
CDN Bundle (incl. Tracing, Replay) - uncompressed	211.5 KB (0%)
CDN Bundle (incl. Tracing, Replay, Feedback) - uncompressed	223.85 KB (0%)
@sentry/nextjs (client)	35.14 KB (0%)
@sentry/sveltekit (client)	33.48 KB (0%)
@sentry/node	114.1 KB (+0.01% 🔺)
@sentry/aws-serverless	103 KB (+0.01% 🔺)

[steebchen]

Thanks so much for this, this is awesome! 🚀