Map http attributes to the stackdriver format

[mayurkale22]

Resolves #651

These changes have all been tested locally against a real stackdriver instance and been confirmed to be working.

Earlier	Now

[mayurkale22]

/cc @loneparadox Please review

[codecov-io]

Codecov Report

Merging #652 into master will increase coverage by 0.2%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master     #652     +/-   ##
=========================================
+ Coverage   95.24%   95.45%   +0.2%     
=========================================
  Files         147      148      +1     
  Lines       10361    10729    +368     
  Branches      792      803     +11     
=========================================
+ Hits         9868    10241    +373     
+ Misses        493      488      -5

Impacted Files	Coverage Δ
src/resource-labels.ts	100% <0%> (ø)	⬆️
test/test-stackdriver-cloudtrace-utils.ts	100% <0%> (ø)	⬆️
test/test-prometheus-stats.ts	100% <0%> (ø)	⬆️
src/validators.ts	100% <0%> (ø)	⬆️
src/detect-resource.ts	100% <0%> (ø)	⬆️
src/constants.ts	100% <0%> (ø)
src/stackdriver-cloudtrace-utils.ts	96.66% <0%> (+0.05%)	⬆️
src/tracecontext-format.ts	95.74% <0%> (+4.83%)	⬆️
test/test-detect-resource.ts	99% <0%> (+5.56%)	⬆️
test/test-tracecontext-format.ts	96.62% <0%> (+7.15%)	⬆️
... and 1 more

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