✨ [RUMF-530][RUM] allow using RUM API before init

[BenoitZugmeyer]

Motivation

We want to allow using RUM APIs before initialising the SDK. (see #545 for the Logs side)

Changes

• Remove stubs and regroup the public API implementation
• Use a bounded buffer to collect pre-init user actions and send them at init
• Make getInternalContext return undefined before init

Testing

Unit tests have been updated. Manual testing can be done by running some DD_RUM.addUserAction('foo') or DD_RUM.getInternalContext() before DD_RUM.init().

---

I have gone over the contributing documentation.

[codecov-commenter]

Codecov Report

Merging #551 into master will decrease coverage by 1.55%.
The diff coverage is 64.28%.

@@            Coverage Diff             @@
##           master     #551      +/-   ##
==========================================
- Coverage   89.66%   88.11%   -1.56%     
==========================================
  Files          38       38              
  Lines        2265     2255      -10     
  Branches      473      474       +1     
==========================================
- Hits         2031     1987      -44     
- Misses        234      268      +34     

Impacted Files	Coverage Δ
packages/core/src/init.ts	70.96% <ø> (-10.86%)	⬇️
packages/rum/src/userActionCollection.ts	98.00% <ø> (ø)
packages/rum/src/rum.ts	77.77% <45.16%> (-14.93%)	⬇️
packages/rum/src/requestCollection.ts	78.37% <66.66%> (-11.87%)	⬇️
packages/rum/src/rum.entry.ts	87.50% <83.33%> (+12.87%)	⬆️
packages/core/src/errorCollection.ts	89.28% <0.00%> (-9.53%)	⬇️
packages/rum/src/rumSession.ts	90.62% <0.00%> (-9.38%)	⬇️
packages/core/src/configuration.ts	86.66% <0.00%> (-5.00%)	⬇️
... and 1 more

---

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[bcaudan]

maybe having a cloneDeep (lodash style) method would be more explicit

[BenoitZugmeyer]

FTR: I'll do this in a separate PR as it has other implications (combine does not perform a deep clone at the moment)

[codecov-io]

Codecov Report

Merging #551 into master will decrease coverage by 0.78%.
The diff coverage is 73.86%.

@@            Coverage Diff             @@
##           master     #551      +/-   ##
==========================================
- Coverage   89.31%   88.52%   -0.79%     
==========================================
  Files          38       39       +1     
  Lines        2265     2258       -7     
  Branches      473      474       +1     
==========================================
- Hits         2023     1999      -24     
- Misses        242      259      +17     

Impacted Files	Coverage Δ
packages/core/src/init.ts	70.96% <ø> (-10.86%)	⬇️
packages/rum/src/lifeCycle.ts	100.00% <ø> (ø)
packages/rum/src/userActionCollection.ts	98.00% <ø> (ø)
packages/rum/src/rum.ts	82.19% <52.77%> (-6.13%)	⬇️
packages/logs/src/logger.ts	95.91% <66.66%> (-4.09%)	⬇️
packages/rum/src/requestCollection.ts	78.37% <66.66%> (-11.87%)	⬇️
packages/rum/src/rum.entry.ts	94.00% <95.83%> (+19.37%)	⬆️
packages/core/src/contextManager.ts	100.00% <100.00%> (ø)
packages/logs/src/logs.entry.ts	100.00% <100.00%> (ø)
... and 7 more

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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