StackDriver Propagator: Make the google header configurable

[nhumrich]

Other google (stackdriver) propagator libraries in open census (for example, python) allow for the header to be configurable. I already have a bunch of projects using a different header. This change allows the header with the trace-context info to be configurable if needed.

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[nhumrich]

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[nhumrich]

@googlebot I signed it!

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[codecov-commenter]

Codecov Report

Merging #818 into master will decrease coverage by 0.50%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #818      +/-   ##
==========================================
- Coverage   97.02%   96.52%   -0.51%     
==========================================
  Files          13        9       -4     
  Lines         841      661     -180     
  Branches       51       69      +18     
==========================================
- Hits          816      638     -178     
+ Misses         25       23       -2     

Impacted Files	Coverage Δ
src/index.ts	100.00% <0.00%> (ø)
src/binary-format.ts
src/resource-labels.ts
src/grpc.ts
test/test-detect-resource.ts
src/grpc-stats/client-stats.ts
src/detect-resource.ts
src/grpc-stats/stats-common.ts
src/grpc-stats/common-distributions.ts
src/grpc-stats/server-stats.ts
... and 11 more

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