feat(rum): add trackWebVitals config to opt out of initial view metrics

[Fiona2016]

Background

A customer integrates the browser SDK into a pre-warmed, hidden Electron BrowserWindow: the window is created with show: false, RUM is initialized immediately, and the actual image content is rendered much later (via IPC) when a preview is needed. RUM then reports an abnormally large LCP.

Root cause

Initial view metrics (FCP / LCP / FID / loading time) are:

• anchored to the page navigation start (clocksOrigin() in trackViews.ts), not to when init() / startView() is called, and
• only collected on the INITIAL_LOAD view.

So when content paints long after navigation, LCP = paint - navigationStart, i.e. huge. The existing safeguards don't catch it:

• trackFirstHidden discards LCP after the page first becomes hidden — but a hidden Electron window does not reliably report document.visibilityState === 'hidden', so the safeguard never triggers.
• the 10-minute LCP_MAXIMUM_DELAY cap only filters truly extreme values.

Deferring init() / using trackViewsManually + manual startView() does not fix it either, because none of those re-baseline timeOrigin — only a real navigation does.

Change

Add a trackWebVitals init option (default true). When false, trackInitialViewMetrics is not started for that SDK instance, so FCP/LCP/FID/loading-time are simply absent for that page rather than reported as a misleading value. This fits the "separate window has its own init" case cleanly — the preview window opts out, the main app is unaffected.

flashcatRum.init({
  applicationId: '...',
  clientToken: '...',
  trackWebVitals: false, // skip FCP/LCP/FID/loading-time on this (pre-warmed/hidden) page
})

Notes

• Like profilingSampleRate and propagateTraceBaggage, this fork-specific option is not part of the upstream telemetry schema, so it is intentionally excluded from serializeRumConfiguration (telemetryEvent.types.ts is generated — "DO NOT MODIFY BY HAND"). The exhaustive serializeRumConfiguration test is updated accordingly.
• Scope is limited to opting out of collection. It does not open the LCP value field in beforeSend (the metric value is deliberately not in the modifiable whitelist).

Tests

• configuration.spec.ts: trackWebVitals defaults to true, honors provided value, casts to boolean.
• trackViews.spec.ts: initial view metrics are not collected when trackWebVitals: false.
• Full unit suite green (2656 passing).

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