Consider adding constants for standard tags

[julienvey]

Use Case

I think the opentracing-go should expose constants for standard opentracing tags, in the same way as the http package exposes constants for status codes. This would be for discoverability, it will also help people new to opentracing, and would also serve as documentation for standard tags.

Proposal

Add constants such as TagComponent = "component" in opentracing package, and update the README/doc to make use of these constants

The link to the opentracing semantic conventions https://github.com/opentracing/specification/blob/master/semantic_conventions.md#standard-span-tags-and-log-fields