[RUMF-617] Extract XHR and Fetch proxies

[bcaudan]

Motivation

Current proxies of XHR and fetch were managed by request collection which was handled as a singleton to avoid to proxy XHR and fetch multiple times.
With the future integration of tracing inside the sdk (cf #461), we would want to collect request with trace id for rum but not for logs.
In a npm setup this would be problematic since the request collection instance would be shared between rum and logs.

Changes

1. Extract XHR and Fetch proxies that:
   • instrument browser API only once
   • deal with request data, not with domain logic (request id, trace id, request type)
   • provide hooks to attach/retrieve info during the request life cycle
2. Use proxies in request collection and error collection
3. Move request collection to rum package

Testing

Automated tests

---

I have gone over the contributing documentation.

[codecov-commenter]

Codecov Report

Merging #468 into master will increase coverage by 0.08%.
The diff coverage is 95.15%.

@@            Coverage Diff             @@
##           master     #468      +/-   ##
==========================================
+ Coverage   87.41%   87.50%   +0.08%     
==========================================
  Files          32       34       +2     
  Lines        2034     2112      +78     
  Branches      411      416       +5     
==========================================
+ Hits         1778     1848      +70     
- Misses        256      264       +8     

Impacted Files	Coverage Δ
packages/rum/src/lifeCycle.ts	100.00% <ø> (ø)
packages/rum/src/matchRequestTiming.ts	93.75% <ø> (ø)
packages/rum/src/rum.ts	86.15% <70.58%> (-0.63%)	⬇️
packages/rum/src/rum.entry.ts	73.21% <90.90%> (+1.51%)	⬆️
packages/rum/src/requestCollection.ts	96.87% <96.87%> (ø)
packages/core/src/fetchProxy.ts	97.72% <97.72%> (ø)
packages/core/src/errorCollection.ts	98.82% <100.00%> (+0.13%)	⬆️
packages/core/src/utils.ts	86.98% <100.00%> (-2.78%)	⬇️
packages/core/src/xhrProxy.ts	100.00% <100.00%> (ø)
packages/logs/src/logs.entry.ts	52.38% <100.00%> (+2.38%)	⬆️
... and 9 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f172792...6bec26c. Read the comment docs.

[BenoitZugmeyer]

LGTM!