message: trace: Fix subscriber benchmarks panicking
  This branch fixes the `tokio-trace` Subscriber benchmarks panicking due
  to constructing spans with ID 0. They will now use an arbitrary constant
  instead.
diff: @@ -18,7 +18,7 @@ struct EnabledSubscriber;
  impl tokio_trace::Subscriber for EnabledSubscriber {
  fn new_span(&self, span: &span::Attributes) -> Id {
  let _ = span;
  - Id::from_u64(0)
  + Id::from_u64(0xDEADFACE)
  }
  fn event(&self, event: &Event) {
  @@ -63,7 +63,7 @@ impl tokio_trace::Subscriber for VisitingSubscriber {
  fn new_span(&self, span: &span::Attributes) -> Id {
  let mut visitor = Visitor(self.0.lock().unwrap());
  span.record(&mut visitor);
  - Id::from_u64(0)
  + Id::from_u64(0xDEADFACE)
  }
  fn record(&self, _span: &Id, values: &span::Record) {
  