[REPLAY-149] implement ViewEnd record

[BenoitZugmeyer]

Motivation

This is part of multitab tracking strategy. To let the player know a view has ended, a new record is added when the view ends or before unloading the page.

Note

There is some limitations with this strategy:

• there may be multiple ViewEnd per view because a ViewEnd record is added even if the "beforeunload" event gets cancelled. We may add a way to detect "beforeunload" cancellations in the future
• there may be no ViewEnd for a view: if the browser fails to send the segment for whatever reason (sendBeacon limitations, connection issues, ...)
• there may be records or segments added after the ViewEnd record: the "beforeunload" event is triggered when the page starts to unload, and it can take a moment (the browser is waiting for the first byte of the next page response to actually leave the current page)

The player will need to be aware of this and handle it gracefully.

Changes

• Adapt the recorder "flush" strategy to avoid sideeffects
• Add a ViewEnd record at the end of views

Testing

Unit + Manual

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I have gone over the contributing documentation.

[codecov-io]

Codecov Report

Merging #711 (c45fcbe) into master (353f6dc) will decrease coverage by 0.10%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #711      +/-   ##
==========================================
- Coverage   79.98%   79.88%   -0.11%     
==========================================
  Files          69       69              
  Lines        3433     3440       +7     
  Branches      732      732              
==========================================
+ Hits         2746     2748       +2     
- Misses        687      692       +5     

Impacted Files	Coverage Δ
packages/rum-core/src/domain/lifeCycle.ts	100.00% <100.00%> (ø)
.../src/domain/rumEventsCollection/view/trackViews.ts	96.74% <100.00%> (+0.02%)	⬆️
packages/rum-recorder/src/boot/recorder.ts	100.00% <100.00%> (ø)
packages/rum-recorder/src/types.ts	100.00% <100.00%> (ø)
packages/rum-core/src/transport/batch.ts	65.71% <0.00%> (-11.43%)	⬇️
...ckages/core/src/domain/automaticErrorCollection.ts	98.41% <0.00%> (-1.59%)	⬇️

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[bcaudan]

LGTM