Change SetTag helpers in ext to public

[blakewedwards]

opentracing-go/ext/tags.go

Line 166 in a7454ce

type stringTagName string

Would it be accepted to make the SetTag helpers like stringTagName public so that users of the package can make their own reusable taggers?

Proposal

ext.stringTagName -> ext.StringTagName
ext.uint16TagName -> ext.Uint16TagName
etc.

[yurishkuro]

I think it'd make sense. The current private types are a bit weird - we return private type, but expect users to use a public method on it. And those methods don't show in godocs

https://godoc.org/github.com/opentracing/opentracing-go/ext

[blakewedwards]

#229