Simplify Logger interface, remove silly, make level optional

[draffensperger]

By removing silly and making the level attribute optional (and defaulted to debug) then the console interface itself will implement Logger. See this TS playground

This will be useful for opencensus-web where a major goal is small code size for web clients, so being able to just use console itself as a logger cuts down on code size.

This does seem like a relatively minor breaking change though.

[justindsmith]

LGTM

[mayurkale22]

Can you please update CHANGELOG with breaking change comment?

[codecov-io]

Codecov Report

Merging #271 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #271      +/-   ##
=========================================
+ Coverage    94.8%   94.8%   +<.01%     
=========================================
  Files         110     110              
  Lines        7779    7764      -15     
  Branches      713     713              
=========================================
- Hits         7375    7361      -14     
+ Misses        404     403       -1

Impacted Files	Coverage Δ
src/common/console-logger.ts	93.1% <0%> (-0.45%)	⬇️
test/test-console-logger.ts	100% <0%> (ø)	⬆️
test/test-stackdriver-monitoring.ts	97.36% <0%> (+0.25%)	⬆️

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[codecov-io]

Codecov Report

Merging #271 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #271      +/-   ##
=========================================
+ Coverage    94.8%   94.8%   +<.01%     
=========================================
  Files         110     110              
  Lines        7779    7764      -15     
  Branches      713     713              
=========================================
- Hits         7375    7361      -14     
+ Misses        404     403       -1

Impacted Files	Coverage Δ
src/common/console-logger.ts	93.1% <0%> (-0.45%)	⬇️
test/test-console-logger.ts	100% <0%> (ø)	⬆️
test/test-stackdriver-monitoring.ts	97.36% <0%> (+0.25%)	⬆️

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[draffensperger]

@mayurkale22 I added a note to the changlog, could you take another look and merge if all is good?