Reproduce bug in stackdriver exporter.

[bogdandrutu]

No description provided.

[songy23]

Please fix the build. You may update https://github.com/census-ecosystem/opencensus-go-exporter-stackdriver/blob/master/metrics_test_utils.go#L45 to ignore the unwanted fields in timestamp pb.

[codecov-io]

Codecov Report

Merging #199 into master will increase coverage by 0.62%.
The diff coverage is 42.85%.

@@            Coverage Diff             @@
##           master     #199      +/-   ##
==========================================
+ Coverage    71.3%   71.92%   +0.62%     
==========================================
  Files          15       15              
  Lines        1582     1585       +3     
==========================================
+ Hits         1128     1140      +12     
+ Misses        374      365       -9     
  Partials       80       80

Impacted Files	Coverage Δ
metrics_test_utils.go	78.94% <42.85%> (-8.56%)	⬇️
metrics_proto.go	85.88% <0%> (+1.8%)	⬆️
stackdriver.go	36.08% <0%> (+5.15%)	⬆️

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