fix(B3Propagator): B3 sampled causing gRPC error

[mayurkale22]

Which problem is this PR solving?

• Fixes plugin-grpc: B3 sampled causing gRPC error #976

• These changes have all been tested locally and been confirmed to be working.

Short description of the changes

• This is what has been done in opentelemetry-python and opentelemetry-go.

[mayurkale22]

/cc @jordanworner would be nice if you can review this change.

[codecov-io]

Codecov Report

Merging #977 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #977   +/-   ##
=======================================
  Coverage   95.02%   95.02%           
=======================================
  Files         209      209           
  Lines        8541     8543    +2     
  Branches      766      768    +2     
=======================================
+ Hits         8116     8118    +2     
  Misses        425      425           

Impacted Files	Coverage Δ
...metry-core/src/context/propagation/B3Propagator.ts	97.61% <100.00%> (+0.11%)	⬆️
...entelemetry-core/test/context/B3Propagator.test.ts	100.00% <100.00%> (ø)
.../opentelemetry-core/test/context/composite.test.ts	96.42% <100.00%> (ø)