feat(factor-outbound-http): report http.response.body.size for WASI p3 responses

[ChihweiLHBird]

Description

Adds the stable OpenTelemetry semantic convention http.response.body.size to the spin_outbound_http.send_request span for outbound HTTP calls made through the WASI p3 interface.

Key points

• The attribute records the actual number of response body bytes delivered to the guest (counted frame-by-frame during streaming), not a Content-Length guess.
• Clean end-of-stream (Ready(None) or is_end_stream()) triggers the recording exactly once via a deferred Span::record on the instrumented span.
• On body read errors the size field is intentionally left unset (to keep the metric untainted) and a warning is emitted with tracing::warn!().

Partially resolve #3188 (Option 2).

[calebschoepp]

Seems fine from an OTel perspective. Maybe @dicej can comment from a P3/HTTP perspective.

[dicej]

LGTM, thanks.

This is a good reminder that we need more test coverage for this kind of thing. I see there is a basic otel_smoke_test in tests/integration.rs, but it doesn't currently do much. Expanding that test to check for e.g. http.response.body.size and make it run both p2 and p3 components seems appropriate.

We might also be able to unit test this kind of thing using e.g. https://crates.io/crates/tracing-fluent-assertions, although I don't personally have experience with that.