Option to emit `url.template` by built-in SocketsHttpHandler tracing and metrics

[antonfirsov]

One conclusion we reached during the discussions around #93221, that it would valuable to emit url.template, which would be a client-side equivalent of http.route: a low-cardinality attribute providing information about the destination endpoint of the request.

Prerequisites for shipping this feature:

• [ ] Stabilize url.template in OTel semconv. This addition likely needs an API and shipping an experimental attribute comes with a significant risk for future breaking changes, which we have to avoid.
• [ ] Conclude the technical design for #110018, since for some users these features are related.

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