Add important event support to the OpenTracing impl

[bg451]

It would be nice to be able to add important annotations via the opentracing API.

I haven't spent much time thinking about it, but it would work by prefixing a tag's key, i.e. span.SetTag("impt:status", resp.StatusCode). This would add an important annotation with key="status".

[emidoots]

Highly agreed, this would be very nice. In another aspect, for the storage backend, one could imagine filtering (indexing) based on 'important' tags like URL route, so that you could search for traces by URL route for example.

I wonder if the distinction between 'important tags' and 'tags' (or 'verbose tags') is a distinction that would be useful to others in the OpenTracing API.. or if it is really only applicable to Appdash? Will need to do some investigation / start up some discussion over there to find out, I guess.