✨ [RUM-246] Support prerendered pages for web vitals

[BeltranBulbarellaDD]

Motivation

Chrome released prerendering support for Chrome 108+ and Edge 108+. This means that all load-related metrics should be relative to when the page was "activated" rather than when it started loading (for non-prerendered pages these values are the same).

Changes

Created interface DocumentWithPrerendering.
Created function getActivationStart which uses the new activationStart.
Adjusted LCP, FCP, TTFB and firstHidden for this new calculation.

Test instructions

All previous tests should pass, and when we mock activation start or have 2 html files where 1 prerenders the other we should see something similar to the image below.

Screen.Recording.2025-06-26.at.10.06.08.mov

Checklist

• [X ] Tested locally
• Tested on staging
• [X ] Added unit tests for this change.
• [X ] Added e2e/integration tests for this change.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 82.00000% with 9 lines in your changes missing coverage. Please review.

Project coverage is 92.14%. Comparing base (65f51ec) to head (5f259db).

Files with missing lines	Patch %	Lines
packages/core/src/browser/browser.types.ts	73.33%	4 Missing ⚠️
...c/domain/view/viewMetrics/trackPrerenderMetrics.ts	78.94%	4 Missing ⚠️
packages/rum-core/src/browser/performanceUtils.ts	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3617      +/-   ##
==========================================
- Coverage   92.21%   92.14%   -0.07%     
==========================================
  Files         326      328       +2     
  Lines        8182     8228      +46     
  Branches     1849     1869      +20     
==========================================
+ Hits         7545     7582      +37     
- Misses        637      646       +9     

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[cit-pr-commenter]

Bundles Sizes Evolution

📦 Bundle Name	Base Size	Local Size	𝚫	𝚫%	Status
Rum	149.17 KiB	150.84 KiB	1.67 KiB	+1.12%	✅
Rum Recorder	17.91 KiB	17.91 KiB	0 B	0.00%	✅
Rum Profiler	4.98 KiB	4.98 KiB	0 B	0.00%	✅
Logs	52.87 KiB	52.87 KiB	0 B	0.00%	✅
Flagging	0 B	931 B	931 B	N/A%	✅
Rum Slim	107.87 KiB	109.50 KiB	1.63 KiB	+1.51%	✅
Worker	23.60 KiB	23.60 KiB	0 B	0.00%	✅

🚀 CPU Performance

Action Name	Base Average Cpu Time (ms)	Local Average Cpu Time (ms)	𝚫
addglobalcontext	0.011	0.012	0.001
addaction	0.025	0.032	0.007
addtiming	0.005	0.007	0.002
adderror	0.023	0.035	0.012
startstopsessionreplayrecording	0.004	0.005	0.001
startview	0.009	0.016	0.007
logmessage	0.023	0.029	0.006

🧠 Memory Performance

Action Name	Base Consumption Memory (bytes)	Local Consumption Memory (bytes)	𝚫 (bytes)
addglobalcontext	28.01 KiB	26.69 KiB	-1356 B
addaction	55.48 KiB	53.46 KiB	-2067 B
addtiming	27.50 KiB	26.24 KiB	-1285 B
adderror	64.35 KiB	61.05 KiB	-3383 B
startstopsessionreplayrecording	27.49 KiB	24.26 KiB	-3309 B
startview	435.27 KiB	426.28 KiB	-9197 B
logmessage	63.04 KiB	99.57 KiB	36.53 KiB

🔗 RealWorld