vortex-io: Add explicit span on spawn_io

vortex-io/src/runtime/handle.rs

@@ -68,7 +68,13 @@ impl Handle {
         R: Send + 'static,
     {
         let (send, recv) = oneshot::channel();
-        let span = tracing::Span::current();
+        // Instrument with a dedicated, named span on its own target rather than
+        // re-entering `Span::current()`. `Instrumented::poll` enters and exits the
+        // span on every poll, so re-entering the caller's span makes its cost scale
+        // with poll count. A distinct target lets subscribers opt these spans in or
+        // out via filtering; when filtered out the span is disabled and enter/exit
+        // are no-ops, which keeps frequently-polled spawned futures cheap.
+        let span = tracing::trace_span!(target: "vortex_io::spawn", "spawn");
         let abort_handle = self.runtime().spawn(
             async move {
                 // Task::detach allows the receiver to be dropped, so we ignore send errors.
@@ -104,7 +110,11 @@ impl Handle {
         R: Send + 'static,
     {
         let (send, recv) = oneshot::channel();
-        let span = tracing::Span::current();
+        // See `spawn` above: a dedicated target rather than `Span::current()` so
+        // subscribers can filter these spans in or out. I/O futures are polled
+        // frequently, so disabling the span (the default for an unconfigured
+        // target) avoids per-poll enter/exit cost.
+        let span = tracing::trace_span!(target: "vortex_io::spawn_io", "spawn_io");
         let abort_handle = self.runtime().spawn_io(
             async move {
                 // Task::detach allows the receiver to be dropped, so we ignore send errors.