tracing: Trace Constellation blocked duration (#34536)

Add a trace for the `select!` macro, so
that we can easily identify the amount of
time we are blocked in the trace
A different option might be to just skip
tracing the `handle_request` function entirely,
since it contains almost nothing interesting.

Signed-off-by: Jonathan Schwender <schwenderjonathan@gmail.com>

components/constellation/constellation.rs

@@ -1206,7 +1206,11 @@ where
         // produces undefined behaviour, resulting in the destructor
         // being called. If this happens, there's not much we can do
         // other than panic.
-        let request = select! {
+        let request = {
+            #[cfg(feature = "tracing")]
+            let _span =
+                tracing::trace_span!("handle_request::select", servo_profiling = true).entered();
+            select! {
                 recv(self.namespace_receiver) -> msg => {
                     msg.expect("Unexpected script channel panic in constellation").map(Request::PipelineNamespace)
                 }
@@ -1238,6 +1242,7 @@ where
                     // where check_timers will be called.
                     return;
                 },
+            }
         };
 
         let request = match request {