message: subscriber: fix bug in new_span for fmt layer
diff: @@ -440,11 +440,8 @@ where
  fn new_span(&self, attrs: &Attributes<'_>, id: &Id, ctx: Context<'_, S>) {
  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<N>>()
  - {
  - let _ = self.fmt_fields.format_fields(fields, attrs);
  - } else {
  +
  + if extensions.get_mut::<FormattedFields<N>>().is_none() {
  let mut buf = String::new();
  if self.fmt_fields.format_fields(&mut buf, attrs).is_ok() {
  let fmt_fields = FormattedFields {
  