Enable test coverage.

[bogdandrutu]

No description provided.

[songy23]

scripts/check-test-files.sh: 4: set: Illegal option -o pipefail
Makefile:28: recipe for target 'test-with-cover' failed
make: *** [test-with-cover] Error 2
The command "make travis-ci" exited with 2.

Please fix.

[bogdandrutu]

@songy23 don't know exactly how. I need to read more details.

[bogdandrutu]

@songy23 for the moment I enabled coverage without the script to check that we don't have empty packages (no tests). I will do that later, but this PR got bigger and a lot of improvements will be blocked.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@dd3a274). Click here to learn what that means.
The diff coverage is 50%.

@@           Coverage Diff            @@
##             master    #200   +/-   ##
========================================
  Coverage          ?   71.3%           
========================================
  Files             ?      15           
  Lines             ?    1582           
  Branches          ?       0           
========================================
  Hits              ?    1128           
  Misses            ?     374           
  Partials          ?      80

Impacted Files	Coverage Δ
monitoredresource/gcp_metadata_config.go	0% <ø> (ø)
resource.go	100% <ø> (ø)
trace.go	59.09% <0%> (ø)
monitoredresource/aws_identity_doc_utils.go	0% <0%> (ø)
stackdriver.go	30.92% <100%> (ø)
stats.go	77.35% <100%> (ø)
metrics_proto.go	84.08% <50%> (ø)
metrics_batcher.go	51.42% <66.66%> (ø)
metrics.go	80.37% <75%> (ø)

---

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[songy23]

Please add an internal/tools.go, similar to https://github.com/open-telemetry/opentelemetry-service/blob/master/internal/tools.go.

[songy23]

LGTM after adding internal/tools.go.