message: subscriber: fix incorrect extensions lookup
  `fmt::Layer::on_record` was looking up extensions using
  `FormattedFields<Self>>` when it should have been using
  `FormattedFields<N>>`.
diff: @@ -342,7 +342,7 @@ where
  let span = ctx.span(id).expect("Span not found, this is a bug");
  let mut extensions = span.extensions_mut();
  if let Some(FormattedFields { ref mut fields, .. }) =
  - extensions.get_mut::<FormattedFields<Self>>()
  + extensions.get_mut::<FormattedFields<N>>()
  {
  let _ = self.fmt_fields.format_fields(fields, values);
  } else {
  