feat: emit element-selector attribution on web-vitals metrics

[Vadman97]

Summary

Switches the SDK's web-vitals listener from the plain web-vitals build to web-vitals/attribution so we can attribute each LCP / CLS / INP / FID to a specific DOM element. The selector and a couple of high-signal context fields are emitted as OTel attributes on the gauge metric, closing a feature parity gap with Datadog RUM's _target_selector.

Attribute keys (kept tight to limit cardinality):

• LCP: web_vital.element, web_vital.attribution.url (sanitized)
• CLS: web_vital.element (largest-shift target), web_vital.load_state
• INP: web_vital.element (event target), web_vital.event_type, web_vital.load_state
• FID: web_vital.element, web_vital.event_type
• FCP: web_vital.load_state
• TTFB: no element-level attribute (timing breakdown is already in the value)

Both consumers (sdk/observe.ts and client/index.tsx) get the new attributes; their existing differing shapes (semconv keys vs. plain group/category) are preserved.

Bundle-size delta (brotli, the enforced metric): 166.07 kB → 166.99 kB (+0.92 kB), well under the 256 kB cap. Gzipped index.umd.js grows by ~1.4 kB.

Test plan

• yarn turbo run build --filter highlight.run
• yarn turbo run test --filter highlight.run (406/406 pass)
• yarn enforce-size (166.99 kB / 256 kB)
• yarn format-check
• Verify in a real page that gauge metrics now carry web_vital.element / web_vital.event_type etc.

🤖 Generated with Claude Code

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Note

Medium Risk
Changes how web-vitals metrics are sourced and adds new per-metric attribution fields, which can affect telemetry shape/cardinality and downstream dashboards. Low security risk but does emit additional (sanitized) URL and DOM-selector data.

Overview
Web-vitals collection now uses web-vitals/attribution and exposes a typed WebVitalMetric union so consumers can safely access per-metric attribution fields.

Both metric emitters (client/index.tsx and sdk/observe.ts) now attach additional web-vitals attributes (e.g. web_vital.element, web_vital.event_type, web_vital.load_state, and a sanitized web_vital.attribution.url) when recording gauges, while keeping each emitter’s existing metric attribute/tag shape intact.

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