feat(ext/telemetry): copy relevant span attributes to HTTP metrics

[bartlomieju]

Summary

Supersedes #28476. Reimplemented from scratch on current main following the same design proposed in #28445.

• Create the HTTP server OTel span in Rust and store it on HttpRecord, so it can be shared with JavaScript
• Expose the span to JS via op_http_get_span — frameworks/users can set attributes like http.route on it
• Before the span ends, op_http_copy_span_attributes_to_otel_info copies http.route from the span to OtelInfo for metrics
• Add http.route to the HTTP server duration histogram and other metric attributes

This enables the workflow described in #28445: a router sets http.route on the auto-instrumented span, and that attribute automatically appears on http.server.request.duration and other HTTP metrics.

Key changes

File	Change
ext/telemetry/lib.rs	Make OtelSpan Clone-able (Rc-wrapped), public; export OtelSpanState, SpanData, SpanContext, etc.
ext/http/service.rs	Create span in handle_request, store on HttpRecord, add get_otel_span + copy_span_attributes_to_otel_info methods
ext/http/http_next.rs	Add op_http_get_span and op_http_copy_span_attributes_to_otel_info ops
ext/http/lib.rs	Add http.route field to OtelInfoAttributes, include in histogram attributes
ext/http/00_serve.ts	Use Rust-created span when available, call copy op before span end
ext/telemetry/telemetry.ts	Export Span class and SPAN_KEY

Closes #28445

Test plan

• Existing otel_basic spec tests pass (HTTP metrics still work)
• Manual test: set http.route on span in handler, verify it appears in metric attributes
• Add spec test for http.route in metrics

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