Add validation for tagkey & tagvalue. Fix issue #268

[vigneshtdev]

Validates tagkey & tagvalues as per the specification given in https://github.com/census-instrumentation/opencensus-specs/blob/master/tags/TagMap.md#tagkey
ran npm test & npm run complie with no errors.

[codecov-io]

Codecov Report

Merging #280 into master will decrease coverage by 0.23%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #280      +/-   ##
==========================================
- Coverage   94.81%   94.57%   -0.24%     
==========================================
  Files         110      106       -4     
  Lines        7771     7434     -337     
  Branches      714      694      -20     
==========================================
- Hits         7368     7031     -337     
  Misses        403      403

Impacted Files	Coverage Δ
test/test-stackdriver-monitoring.ts	97.1% <0%> (-0.26%)	⬇️
src/metrics/gauges/derived-gauge.ts	96.42% <0%> (-0.07%)	⬇️
src/metrics/gauges/gauge.ts	100% <0%> (ø)	⬆️
test/test-console-logger.ts	100% <0%> (ø)	⬆️
test/test-derived-gauge.ts	100% <0%> (ø)	⬆️
test/test-metric-producer.ts	100% <0%> (ø)	⬆️
test/test-view.ts	99.2% <0%> (ø)	⬆️
test/test-gauge.ts	100% <0%> (ø)	⬆️
src/metrics/export/metric-producer-manager.ts	100% <0%> (ø)	⬆️
test/test-metric-registry.ts	100% <0%> (ø)	⬆️
... and 6 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 239a9e2...0bc2a1d. Read the comment docs.

[mayurkale22]

I think we should log the warn or error, in case of invalid tags. WDYT?

[vigneshtdev]

How about modifying the method like this:

private invalidTags(tags: Tags): boolean {
  const result: boolean =  this.invalidPrintableCharacters(tags) || this.invalidLength(tags);
  if(result) {
    console.log('Unable to create tagkey/tagvalue with the specified tags. Check Tag specifications here: https://github.com/census-instrumentation/opencensus-specs/blob/master/tags/TagMap.md');
  }
  return result;
}

[mayurkale22]

s/console.log/this.logger.warn and no need to add specs link.

[mayurkale22]

Thanks @vigneshtdev for contribution... 👍