OpenCensus Web B3 Propagation

[edvinasbartkus]

Creates a new web package called opencensus-web-b3-propagation.
The way web b3 propagation works is identical to nodejs/b3-propagation. In this package, it includes web-core for internal references (for example for random trace id generation).

startTracing is changed to accept options = { propagation: Propagation }.

By default opencensus/web-propagation-tracecontext.TraceContextFormat will be used but it can be replaced with opencensus/web-propagation-b3.B3Format

@draffensperger am I on the right path? :)

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[edvinasbartkus]

@googlebot I signed it!

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[codecov-io]

Codecov Report

Merging #173 into master will decrease coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #173      +/-   ##
=========================================
- Coverage   95.79%   95.7%   -0.09%     
=========================================
  Files          30      32       +2     
  Lines         857     909      +52     
  Branches      148     162      +14     
=========================================
+ Hits          821     870      +49     
- Misses         36      39       +3

Impacted Files	Coverage Δ
src/xhr-interceptor.ts	100% <0%> (ø)	⬆️
src/b3-format.ts	90.9% <0%> (ø)
src/validators.ts	100% <0%> (ø)
src/interaction-tracker.ts	96.46% <0%> (+0.12%)	⬆️

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[draffensperger]

This is awesome!! I just have one small nit, but otherwise LGTM. I will approve once we fix that comment.