message: subscriber: fix missing `register_callsite` for `Box<dyn Subscribe>`
  Turns out the `Subscribe` impl for `Box<dyn Subscribe<...> + ...>` never
  called the subscriber trait object's `register_callsite` method. Whoops!
  This commit adds the missing `register_callsite` hook.
diff: @@ -1397,6 +1397,11 @@ macro_rules! subscriber_impl_body {
  self.deref_mut().on_subscribe(collect);
  }
  + #[inline]
  + fn register_callsite(&self, metadata: &'static Metadata<'static>) -> Interest {
  + self.deref().register_callsite(metadata)
  + }
  +
  #[inline]
  fn on_new_span(&self, attrs: &span::Attributes<'_>, id: &span::Id, ctx: Context<'_, C>) {
  self.deref().on_new_span(attrs, id, ctx)
  