Preserve the tracing span in the EndpointDriver task

The EndpointDriver is spawned in a task.  Unless the span is copied
into this task the current span is lost.

Copying the current span means any tracing calls inside the driver,
from e.g. a custom AsyncUdpSocket, are preserved in the span.  This
makes it easy to use different parent spans for different endpoints
and have log messages appear with their parent span.  Otherwise these
log messages are not attached to a span.

quinn/src/endpoint.rs

@@ -20,6 +20,7 @@ use proto::{
 };
 use rustc_hash::FxHashMap;
 use tokio::sync::{futures::Notified, mpsc, Notify};
+use tracing::{Instrument, Span};
 use udp::{RecvMeta, BATCH_SIZE};
 
 use crate::{
@@ -114,11 +115,14 @@ impl Endpoint {
             runtime.clone(),
         );
         let driver = EndpointDriver(rc.clone());
-        runtime.spawn(Box::pin(async {
-            if let Err(e) = driver.await {
-                tracing::error!("I/O error: {}", e);
+        runtime.spawn(Box::pin(
+            async {
+                if let Err(e) = driver.await {
+                    tracing::error!("I/O error: {}", e);
+                }
             }
-        }));
+            .instrument(Span::current()),
+        ));
         Ok(Self {
             inner: rc,
             default_client_config: None,