Change CLS import to make stats types more general. (Fix #527)

[crdgonzalezca]

This PR fixes Issue #527.

• Adds a Default function declaration in @opencensus/core stats/types.ts to avoid import of internal functionality.
• As small change, adds missing semicolon.

[codecov-io]

Codecov Report

Merging #528 into master will increase coverage by 0.26%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #528      +/-   ##
==========================================
+ Coverage   94.89%   95.15%   +0.26%     
==========================================
  Files         146      147       +1     
  Lines        9379    10547    +1168     
  Branches      537      912     +375     
==========================================
+ Hits         8900    10036    +1136     
- Misses        479      511      +32

Impacted Files	Coverage Δ
src/http2.ts	80.64% <0%> (-8.65%)	⬇️
test/test-instana.ts	85.18% <0%> (-3.28%)	⬇️
src/trace/model/tracer.ts	66.66% <0%> (-3.11%)	⬇️
src/trace/instrumentation/base-plugin.ts	9.52% <0%> (-1.59%)	⬇️
test/test-http2.ts	96.81% <0%> (-1.22%)	⬇️
test/test-zpages.ts	99.27% <0%> (-0.36%)	⬇️
src/internal/string-utils.ts	100% <0%> (ø)	⬆️
src/http-stats.ts	100% <0%> (ø)	⬆️
src/tags/tag-map.ts	100% <0%> (ø)	⬆️
test/test-tag-map.ts	100% <0%> (ø)	⬆️
... and 100 more

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[mayurkale22]

@crdgonzalezca Thanks for the issue and PR.