[RUM-260] Support bfcache restore for web vitals

[RomanGaignault]

Motivation

Users navigating back to a page via the browser’s back-forward cache do not experience a full page reload. Therefore, initial performance metrics may no longer accurately represent what the user sees upon restoration. By capturing metrics specifically for bfCache restores, we gain a more precise understanding of user-perceived performance.

Changes

This PR introduces support for detecting and handling the Back-Forward Cache (bfCache). When a page is restored from bfCache (i.e., a pageshow event with persisted = true)

Testing

• Local
• Staging
• Unit
• End to end

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I have gone over the contributing documentation.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.04%. Comparing base (faa6d5e) to head (2a94df5).
Report is 14 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3357      +/-   ##
==========================================
+ Coverage   93.02%   93.04%   +0.02%     
==========================================
  Files         298      300       +2     
  Lines        7853     7883      +30     
  Branches     1789     1790       +1     
==========================================
+ Hits         7305     7335      +30     
  Misses        548      548              

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[cit-pr-commenter]

Bundles Sizes Evolution

📦 Bundle Name	Base Size	Local Size	𝚫	𝚫%	Status
Rum	147.69 KiB	148.36 KiB	688 B	+0.45%	✅
Logs	51.65 KiB	51.65 KiB	0 B	0.00%	✅
Rum Slim	106.42 KiB	107.08 KiB	682 B	+0.63%	✅
Worker	24.50 KiB	24.50 KiB	0 B	0.00%	✅

🚀 CPU Performance

Action Name	Base Average Cpu Time (ms)	Local Average Cpu Time (ms)	𝚫
addglobalcontext	0.002	0.003	0.001
addaction	0.038	0.058	0.020
addtiming	0.001	0.001	0.000
adderror	0.051	0.072	0.022
startstopsessionreplayrecording	0.007	0.011	0.004
startview	0.394	0.526	0.131
logmessage	0.023	0.027	0.004

🧠 Memory Performance

Action Name	Base Consumption Memory (bytes)	Local Consumption Memory (bytes)	𝚫 (bytes)
addglobalcontext	31.47 KiB	28.84 KiB	-2697 B
addaction	58.93 KiB	55.51 KiB	-3508 B
addtiming	32.09 KiB	26.42 KiB	-5805 B
adderror	63.03 KiB	56.94 KiB	-6239 B
startstopsessionreplayrecording	25.77 KiB	25.51 KiB	-263 B
startview	423.91 KiB	424.02 KiB	108 B
logmessage	60.48 KiB	57.98 KiB	-2562 B

🔗 RealWorld

[RomanGaignault]

/to-staging

[dd-devflow]

View all feedbacks in Devflow UI.
2025-02-20 21:11:27 UTC ℹ️ Start processing command /to-staging

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2025-02-20 21:11:31 UTC ℹ️ Branch Integration: starting soon, median merge time is 13m38s

Commit 2a94df54cb will soon be integrated into staging-08.

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2025-02-20 21:44:19 UTC ℹ️ Branch Integration: This commit was successfully integrated

Commit 2a94df54cb has been merged into staging-08 in merge commit 3291b7c64b.

Check out the triggered pipeline on Gitlab 🦊

If you need to revert this integration, you can use the following command: /code revert-integration -b staging-08