runtime: Instrument with tracing

[brancz]

Summary

This is so that tracing subscribers, can correctly attribute information to eg. opentelemetry distributed tracing IDs.

API Changes

None

Testing

Wrote a test, but didn't check it in because it would require the std feature of tracing and make tracing-subscriber a dependency. Happy to add the test if those things are ok.

    #[tokio::test]
    async fn test_spawn_propagates_tracing_span() {
        install_global_subscriber();
        let handle = TokioRuntime::current();

        let span = tracing::info_span!("parent_span");
        let parent_id = span.id();
        assert!(parent_id.is_some(), "subscriber must be active");
        let _enter = span.enter();

        // spawn: async future is instrumented with the parent span
        let spawned_id = handle.spawn(async { tracing::Span::current().id() }).await;
        assert_eq!(spawned_id, parent_id, "spawn should propagate span");

        // spawn_cpu: closure enters the parent span
        let cpu_id = handle.spawn_cpu(|| tracing::Span::current().id()).await;
        assert_eq!(cpu_id, parent_id, "spawn_cpu should propagate span");

        // spawn_blocking: closure enters the parent span on the blocking thread
        let blocking_id = handle
            .spawn_blocking(|| tracing::Span::current().id())
            .await;
        assert_eq!(
            blocking_id, parent_id,
            "spawn_blocking should propagate span"
        );
    }

[AdamGS]

I'm not sure there's a test that will make me super confident that tracing is correctly set up in async rust 😅 code is simple enough and seems correct to me.