implement redis instrumentation plugin

[vmarchaud]

Not sure if everything is okay it terms of setup since i don't use lerna a lot, same with circleCI.
I've set the version to 0.0.1 but i believe you guys have all the packages at the same version for now, feel free to do it as you want.
I also choose not to add it into the instrumentation-all packages directly too, but i could do another PR if the plugin in itself is good.

cc @draffensperger @mayurkale22

[codecov-io]

Codecov Report

Merging #359 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #359      +/-   ##
==========================================
- Coverage   95.23%   95.22%   -0.02%     
==========================================
  Files         129      131       +2     
  Lines        8592     8758     +166     
  Branches      634      638       +4     
==========================================
+ Hits         8183     8340     +157     
- Misses        409      418       +9

Impacted Files	Coverage Δ
src/stackdriver-monitoring.ts	81.05% <0%> (-2.11%)	⬇️
src/redis.ts	93.54% <0%> (ø)
test/test-redis.ts	97.11% <0%> (ø)

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[draffensperger]

This is awesome, thanks for taking the initiative on this!

I have a bunch of nit-picky comments, but overall this is great.

[mayurkale22]

Thanks for the contribution, I have added a few comments.

[vmarchaud]

I believe i addressed every comment, note that the test failures are related to Stackdriver and not the PR itself

[mayurkale22]

Please rebase the PR, this should fix build issue.

[mayurkale22]

This is good to go, please rebase once.

[vmarchaud]

@mayurkale22 Done

[mayurkale22]

Thanks for the contribution again! If possible, please include redis instrumentation example in examples package.

[vmarchaud]

Yes, no problem. I will try to make another PR with the support for ioredis, and add both in examples and in opencensus-instrumentation-all