message: Refactor entry of spans in core reactor function
diff: @@ -552,8 +552,7 @@ where
  // Create another span for tracing the processing of one event.
  let event_span = debug_span!("dispatch events", ev = self.event_count);
  - let inner_enter = event_span.enter();
  -
  + let (effects, keep_going) = event_span.in_scope(|| {
  // We log events twice, once in display and once in debug mode.
  let event_as_string = format!("{}", event);
  debug!(event=%event_as_string, ?q);
  @@ -592,13 +591,11 @@ where
  .event_dispatch_duration
  .observe(delta.into_nanos() as f64);
  - drop(inner_enter);
  -
  - // We create another span for the effects, but will keep the same ID.
  - let effect_span = debug_span!("process effects", ev = self.event_count);
  + (effects, keep_going)
  + });
  process_effects(self.scheduler, effects)
  - .instrument(effect_span)
  + .instrument(debug_span!("process effects", ev = self.event_count))
  .await;
  self.event_count += 1;
  