✨ [RUMF-1467] Collect user data telemetry

[amortemousque]

Motivation

Monitor user usages to understand the user data volume and its impact on our batch strategy:

• Measure the number of events per batch
• Measure contexts bytes volumes per batch (global context, user context, feature flags)

Worth to review commit by commit

Changes

• Move computeBytesCount from batch to utils
• Count the context bytes of ContextManager
• Count the FeatureFlagContext bytes
• Make the Batch flush observable
• Collect user data telemetry

Testing

• Local
• Staging
• Unit
• End to end

---

I have gone over the contributing documentation.

[codecov-commenter]

Codecov Report

Merging #1941 (242b255) into main (90eee94) will increase coverage by 0.04%.
The diff coverage is 96.19%.

@@            Coverage Diff             @@
##             main    #1941      +/-   ##
==========================================
+ Coverage   93.12%   93.16%   +0.04%     
==========================================
  Files         142      144       +2     
  Lines        5407     5484      +77     
  Branches     1228     1248      +20     
==========================================
+ Hits         5035     5109      +74     
- Misses        372      375       +3     

Impacted Files	Coverage Δ
packages/rum-core/src/domain/configuration.ts	93.87% <ø> (ø)
packages/rum-core/src/domain/lifeCycle.ts	100.00% <ø> (ø)
...ain/rumEventsCollection/action/actionCollection.ts	95.45% <ø> (ø)
...omain/rumEventsCollection/error/errorCollection.ts	100.00% <ø> (ø)
...rum-core/src/domain/contexts/featureFlagContext.ts	92.30% <81.81%> (-7.70%)	⬇️
packages/core/src/tools/utils.ts	81.92% <85.71%> (+0.10%)	⬆️
.../rum-core/src/domain/startCustomerDataTelemetry.ts	98.07% <98.07%> (ø)
packages/core/src/domain/telemetry/telemetry.ts	96.22% <100.00%> (+0.07%)	⬆️
packages/core/src/tools/contextManager.ts	100.00% <100.00%> (ø)
packages/core/src/transport/batch.ts	97.26% <100.00%> (+1.10%)	⬆️
... and 8 more

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[bcaudan]

Great!