fix: run dns resolution in the same tracing-span as the caller

This makes it possible to trace the entire request, including DNS
resolution.

The use case for this is to be able to suppress specific requests from
being traced in a situation where the request is used to transmit logs
to a remote server. This would result in an infinite loop of logs being
sent to the remote server. tokio-rs/tokio#6659
has more details.

src/client/legacy/connect/dns.rs

@@ -118,7 +118,9 @@ impl Service<Name> for GaiResolver {
     }
 
     fn call(&mut self, name: Name) -> Self::Future {
+        let current_span = tracing::Span::current();
         let blocking = tokio::task::spawn_blocking(move || {
+            let _enter = current_span.enter();
             debug!("resolving host={:?}", name.host);
             (&*name.host, 0)
                 .to_socket_addrs()