Remove unnecessary URL construction

[trask]

Constructing URL objects is a bit costly (in addition to memory, it does a lot of validation), and we construct them in various http server and http client instrumentations, generally in order to populate http.url attribute. This could probably be optimized by adding some kind of "uri builder" to our api that can be used instead.

[anuraaga]

I think the intent of the semantic conventions is to only return a URL if we have a URL, or otherwise return the parts, target, host, etc. Though not sure if this just makes it too confusing when writing instrumentation.