Stats: apply restriction on TagKey and TagValue

[mayurkale22]

The TagKey name must meet the following requirements:

1. Must contain only printable ASCII (codes between 32 and 126 inclusive)
2. Must have length greater than zero and less than 256.
3. Must not be empty.

The TagValue name must meet the following requirements:

1. It MUST contain only printable ASCII (codes between 32 and 126)

Specs: https://github.com/census-instrumentation/opencensus-specs/blob/master/tags/TagMap.md#tagkey

[vigneshtdev]

hey i would like to help with this issue. In opencensus java it had a package called tag where different tagvalue & tagkey can be configured. Where should I look for it in this package?

[mayurkale22]

@vigneshtdev Thanks for your interest in the project, all contributions are welcomed and highly appreciated. Currently we dont have separate package for Tags api. I think we should create new one same as Java and Python. WDYT?

[vigneshtdev]

I think that it is the correct way to go, as it will make implementation across all languages more consistent. However it would require major refactoring in both test & soruce

[vigneshtdev]

Or else if such refactoring in unnecessary we can modify the existing invalidTags method. Currenlty it only check if the tagkey & tagvalue have printable characters.

[songy23]

Fixed in #280.