Codecov: start reporting code coverage.

[mayurkale22]

This PR starts reporting code coverage. If all goes well, coverage should be at https://codecov.io/gh/census-instrumentation/opencensus-node

For now, I have considered four packages (nodejs, core, exporter-prometheus and exporter-stackdriver) for codecov. If all goes well, I will expand this for other packages.

[codecov-io]

Codecov Report

Merging #232 into master will increase coverage by 13.76%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master     #232       +/-   ##
===========================================
+ Coverage   81.64%   95.41%   +13.76%     
===========================================
  Files          12       58       +46     
  Lines         376     3841     +3465     
  Branches       62      276      +214     
===========================================
+ Hits          307     3665     +3358     
- Misses         48      176      +128     
+ Partials       21        0       -21

Impacted Files	Coverage Δ
src/exporters/console-exporter.ts	77.77% <0%> (-22.23%)	⬇️
src/trace/model/span-base.ts	100% <0%> (ø)	⬆️
src/internal/string-utils.ts	100% <0%> (ø)
src/index.ts	100% <0%> (ø)
src/stats/metric-producer.ts	100% <0%> (ø)
src/prometheus-stats.ts	94.18% <0%> (ø)
test/test-metric-utils.ts	100% <0%> (ø)
test/nocks.ts	63.26% <0%> (ø)
src/stats/types.ts	100% <0%> (ø)
test/test-console-logger.ts	100% <0%> (ø)
... and 48 more

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[justindsmith]

Looks good over all. Just for my own clarification, this will report on every PR update as well as merge, right?

[mayurkale22]

Just for my own clarification, this will report on every PR update as well as merge, right?

I am not sure about merge, but yes on every PR update.