Use global API instances

[dyladan]

As discussed in the SIG and on issue #885, this uses the global object to store references to the global APIs. When an API method is called, it tries to get the global API instance by calling a method with its API version. If the version matches, the call is properly proxied through to the API, else it calls a no-op version.

[codecov-io]

Codecov Report

Merging #943 into master will decrease coverage by 0.05%.
The diff coverage is 78.94%.

@@            Coverage Diff             @@
##           master     #943      +/-   ##
==========================================
- Coverage   94.95%   94.90%   -0.06%     
==========================================
  Files         210      212       +2     
  Lines        8571     8676     +105     
  Branches      772      791      +19     
==========================================
+ Hits         8139     8234      +95     
- Misses        432      442      +10     

Impacted Files	Coverage Δ
...ckages/opentelemetry-web/test/registration.test.ts	0.00% <0.00%> (ø)
packages/opentelemetry-api/src/api/propagation.ts	46.66% <23.07%> (-10.48%)	⬇️
packages/opentelemetry-api/src/api/metrics.ts	47.61% <33.33%> (-5.72%)	⬇️
packages/opentelemetry-api/src/api/context.ts	75.00% <75.00%> (+27.94%)	⬆️
packages/opentelemetry-api/src/api/trace.ts	85.71% <90.00%> (-0.96%)	⬇️
packages/opentelemetry-api/src/api/global-utils.ts	100.00% <100.00%> (ø)
packages/opentelemetry-api/test/api/api.test.ts	96.96% <100.00%> (+0.30%)	⬆️
packages/opentelemetry-api/test/api/global.test.ts	100.00% <100.00%> (ø)
...opentelemetry-node/test/NodeTracerProvider.test.ts	100.00% <100.00%> (ø)
...kages/opentelemetry-node/test/registration.test.ts	100.00% <100.00%> (ø)
... and 10 more

[vmarchaud]

lgtm, i have just one question concerning the api version for each api, how do we bump them ? For each major ?

[dyladan]

We bump them whenever we make a change that is not backwards compatible. So adding a method would not require a version bump, as an old version of the API module would simply not call that method. Removing a method that may be called by an older version of the API module would require a version bump.

[obecny]

Added few comments.
This is just a suggestion as I don't have the whole view on the spec. But few things are repeated (setting, getting, disabling) maybe it makes sense to make some helper function / functions or class for that so the code can be reused and tested in one place ? The difference is only with global key and the returned type, so there could be some similarities. Are they going to be more globals in future too, which should be implemented the same way then ?

[obecny]

lgtm, thx for those changes

[dyladan]

@mayurkale22 would appreciate if you can take another look at this