Silently skip NO_ERROR Goaway

[songy23]

Closes #869.

Based on the discussion in #869, I think it's safe to silently skip status UNAVAILABLE with error code NO_ERROR.

[coveralls]

Coverage decreased (-0.07%) to 84.123% when pulling 4daf1d0 on songy23:no-error-goaway into 7720668 on census-instrumentation:master.

[coveralls]

Coverage decreased (-0.1%) to 84.068% when pulling 4daf1d0 on songy23:no-error-goaway into 7720668 on census-instrumentation:master.

[sebright]

Should the exporter retry in this case?

[songy23]

I'm hesitant on doing retry in this case, since I think NO_ERROR Goaway indicates server should have received and processed the request with no error. Retrying might generate duplicated stats and increase the burden of server.

[codecov-io]

Codecov Report

Merging #935 into master will decrease coverage by 0.19%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##             master    #935     +/-   ##
==========================================
- Coverage      81.8%   81.6%   -0.2%     
  Complexity     1032    1032             
==========================================
  Files           169     169             
  Lines          4600    4611     +11     
  Branches        431     434      +3     
==========================================
  Hits           3763    3763             
- Misses          730     741     +11     
  Partials        107     107

Impacted Files	Coverage Δ	Complexity Δ
...r/stats/stackdriver/StackdriverExporterWorker.java	61.86% <0%> (-6.36%)	13 <0> (ø)

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[coveralls]

Coverage decreased (-0.1%) to 84.049% when pulling f06a1b7 on songy23:no-error-goaway into 7720668 on census-instrumentation:master.

[coveralls]

Coverage decreased (-0.2%) to 83.995% when pulling 16afb30 on songy23:no-error-goaway into fc71453 on census-instrumentation:master.

[coveralls]

Coverage decreased (-0.2%) to 83.995% when pulling a4990bd on songy23:no-error-goaway into fc71453 on census-instrumentation:master.

[songy23]

Talked offline. I'll close this PR for now, until I can make sure this failure has no impact on the uploaded stats.