Add content-length header attributes to outbound HTTP spans

[ChihweiLHBird]

Description

Records http.request.header.content-length and http.response.header.content-length on outbound HTTP spans when the header is present.

Uses set_attribute() from tracing_opentelemetry::OpenTelemetrySpanExt because the OTel attribute names contain hyphens, which tracing's #[instrument] field system cannot represent.

I didn't implement a test because I thought it's not worth to add another expensive integration test case just for a new field sent to OTel collector.

I used a small HTTP component and otel-desktop-viewer to verify this works as expected:

#[http_component]
async fn handle_test_spin_rust(req: Request) -> anyhow::Result<impl IntoResponse> {
    println!("Handling request to {:?}", req.header("spin-full-url"));

    let outbound_request = Request::get("https://httpbin.org/get")
        .header("accept", "application/json")
        .build();
    let outbound_response: Response = spin_sdk::http::send(outbound_request)
        .await
        .context("failed to call https://httpbin.org/get")?;
    let outbound_status = outbound_response.status();
    let outbound_body_len = outbound_response.body().len();
    
    Ok(Response::builder()
        .status(200)
        .header("content-type", "text/plain")
        .body(format!(
            "Hello World!\nhttpbin.org/get status: {outbound_status}\nhttpbin.org/get body bytes: {outbound_body_len}"
        ))
        .build())
}

Addresses part of #3188 (Option 1).

Changes

• crates/factor-outbound-http/Cargo.toml — added tracing-opentelemetry dependency
• crates/factor-outbound-http/src/wasi.rs — added record_content_length_header helper and call sites for request (after interceptor) and response

[calebschoepp]

This change seems reasonable to me. LGTM