Add support for gRPC tags propagation

[mayurkale22]

Fixes #325

This PR contains the following changes

1. Share same contextManager across tracer and stats singleton instances.
2. Add withTagContext and getCurrentTagContext API on stats, so that plugins can use these API to get current TagContext/TagMap.
3. Make a deep copy of TagMap during get/set into contextManager.
4. Insert/Get gRPC metadata with grpc-tags-bin header with serialized TagMap.
5. Update tag-context-example.js example.

[codecov-io]

Codecov Report

Merging #443 into master will increase coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #443      +/-   ##
=========================================
+ Coverage   95.02%   95.1%   +0.07%     
=========================================
  Files         149     149              
  Lines        9655    9760     +105     
  Branches      683     690       +7     
=========================================
+ Hits         9175    9282     +107     
+ Misses        480     478       -2

Impacted Files	Coverage Δ
src/index.ts	100% <0%> (ø)	⬆️
src/stats/types.ts	100% <0%> (ø)	⬆️
src/tags/tagger.ts	100% <0%> (ø)	⬆️
test/test-tagger.ts	100% <0%> (ø)	⬆️
test/test-grpc.ts	99.49% <0%> (+0.11%)	⬆️
src/stats/stats.ts	98.27% <0%> (+0.19%)	⬆️
src/grpc.ts	94.11% <0%> (+0.39%)	⬆️
src/internal/cls-ah.ts	53.96% <0%> (+2.35%)	⬆️
src/internal/cls.ts	91.66% <0%> (+9.84%)	⬆️

---

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 b5a49c1...9b62e2b. Read the comment docs.

[draffensperger]

Cool! It looks like the grpc-tags-bin header is only being serialized/deserialized in the getTagContext/setTagContext helper functions. Are those called automatically when gRPC calls happen? (Or will that be in a follow-up CL?)

[mayurkale22]

Cool! It looks like the grpc-tags-bin header is only being serialized/deserialized in the getTagContext/setTagContext helper functions. Are those called automatically when gRPC calls happen? (Or will that be in a follow-up CL?)

I have added hook to use these functions during gRPC incoming request and outgoing request .

[draffensperger]

Cool, nice that the gRPC plugins now support tags with those new changes.