✨ [RUMF-530][Logs] allow using logs API before init

[BenoitZugmeyer]

Motivation

We want to allow using log APIs before initialising the SDK.

Changes

• Remove stubs and regroup the public API implementation
• Introduce a 'buffered function' to buffer pre-ini function calls
• Use a buffered function to send logs

Testing

Unit tests have been updated. Manual testing can be done by running some DD_LOGS.logger.log('foo') before DD_LOGS.init().

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I have gone over the contributing documentation.

[codecov-commenter]

Codecov Report

Merging #545 into master will increase coverage by 1.55%.
The diff coverage is 91.20%.

@@            Coverage Diff             @@
##           master     #545      +/-   ##
==========================================
+ Coverage   88.11%   89.66%   +1.55%     
==========================================
  Files          36       38       +2     
  Lines        2288     2265      -23     
  Branches      469      473       +4     
==========================================
+ Hits         2016     2031      +15     
+ Misses        272      234      -38     

Impacted Files	Coverage Δ
packages/core/src/configuration.ts	91.66% <ø> (-0.27%)	⬇️
packages/logs/src/logs.ts	74.19% <74.19%> (ø)
packages/core/src/boundedBuffer.ts	100.00% <100.00%> (ø)
packages/core/src/init.ts	81.81% <100.00%> (+1.81%)	⬆️
packages/logs/src/logger.ts	100.00% <100.00%> (+4.59%)	⬆️
packages/logs/src/logs.entry.ts	100.00% <100.00%> (+50.74%)	⬆️
packages/rum/src/rum.entry.ts	74.62% <100.00%> (ø)
packages/rum/src/performanceCollection.ts	68.57% <0.00%> (+1.42%)	⬆️
... and 1 more

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[bcaudan]

Nice job 👍

[tbillington]

Just letting you know this is a breaking change but wasn't reported as such in your changelog.

[bcaudan]

👋 @tbillington, it should not be.
There was an issue in the logs documentation, isCollectingError was not an init parameter for logs, forwardErrorToLogs was the right one: https://github.com/DataDog/browser-sdk/pull/545/files#diff-2e4bca1144de53c8c504edb040ac34aa7ea63ba413a4453277d88307278d8972R43.
It is still collecting error by default: https://github.com/DataDog/browser-sdk/pull/545/files#diff-c62c675364c11388aeae30517afdac97f57f2638e8fb3d019177ebfeb382f540R25

If you still see something wrong, would you mind opening a dedicated issue and provide more details?