✨ [RUMF-775] implement Largest Contentful Paint

[BenoitZugmeyer]

Motivation

Expose the web-vitals LCP on initial views

Changes

• Implement LCP tracking
• Use it on initial views

Testing

CI + manual tests on chrome

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I have gone over the contributing documentation.

[codecov-io]

Codecov Report

Merging #624 (b43b54b) into master (de06363) will increase coverage by 0.02%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master     #624      +/-   ##
==========================================
+ Coverage   88.23%   88.25%   +0.02%     
==========================================
  Files          56       56              
  Lines        2525     2538      +13     
  Branches      546      547       +1     
==========================================
+ Hits         2228     2240      +12     
- Misses        297      298       +1     

Impacted Files	Coverage Δ
packages/rum/src/browser/performanceCollection.ts	22.72% <0.00%> (-0.35%)	⬇️
.../domain/rumEventsCollection/view/viewCollection.ts	100.00% <ø> (ø)
packages/rum/src/typesV2.ts	100.00% <ø> (ø)
packages/core/src/tools/utils.ts	86.74% <100.00%> (+0.08%)	⬆️
...rc/domain/rumEventsCollection/view/trackTimings.ts	100.00% <100.00%> (ø)
packages/rum/src/transport/batch.ts	78.04% <0.00%> (ø)

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[bcaudan]

it could be helpful to add a link to the spec of these new timings or maybe describe the different rules in a function comment.

[BenoitZugmeyer]

I can't find a link to show that it is needed. All the documentation (here, spec draft readme, spec draft ) seems to agree that the browser won't send those performance entries after the first user interaction. I think that's just a safeguard that web-vitals implemented because of some Chrome issues.