message: subscriber: Forward current_span impl for Layered
  This overrides the default `Subscriber::current_span` impl
  with one that forwards the inner subscribers implementation.
  This fixes bugs where the current_span was not fetchable when
  using the `FmtSubscriber`.
diff: @@ -501,6 +501,11 @@ where
  }
  }
  + #[inline]
  + fn current_span(&self) -> span::Current {
  + self.inner.current_span()
  + }
  +
  #[doc(hidden)]
  unsafe fn downcast_raw(&self, id: TypeId) -> Option<*const ()> {
  self.layer
  