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 Customizable Uri redaction for HTTP logs and traces #110018, since for some users these features are related.

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