feat(tracing): add generated metadata spans - #3269

Summary

• emit Next.js-compatible ResolveMetadata.generateMetadata spans through the shared framework tracer
• preserve exact module identities for pages, layouts, nested parallel slots, intercepts, and HTTP-access fallback conventions
• move App Page element preparation inside AppRender.getBodyResult so generated metadata and application child spans have the same parent as Next.js
• prove the hierarchy with an unchanged real @sentry/nextjs integration on Workerd

Next.js parity

Pinned against vercel/next.js@b421cadefd31c1b59d117842021ded7c1ebaf5b4, especially:

• packages/next/src/lib/metadata/metadata-resolution-primitives.ts
• packages/next/src/lib/metadata/resolve-metadata.ts
• packages/next/src/server/app-render/app-render.tsx
• test/e2e/opentelemetry/instrumentation/opentelemetry.test.ts

The trace route excludes parallel-slot directory keys, retains route groups/interception segments, and uses not-found, forbidden, or unauthorized plus the owning module route. Static metadata exports remain untraced.

No OpenTelemetry package is added as a required vinext dependency, and no Sentry-specific runtime path is introduced. The same descriptors flow to Workers tracing automatically.

Validation

• vp check
• focused metadata/render/head/boundary suites (270 passed)
• vp run vinext#build
• PLAYWRIGHT_PROJECT=cloudflare-sentry-app pnpm run test:e2e (17 passed)
• independent correctness review: clean after addressing active/inactive nested slot route identity without changing behavioral metadata routes
• independent Next.js parity review: clean after addressing fallback convention, owner route, and @slot identity findings

Stack parent: #3268