message: subscriber: fix `clippy::map_flatten` lint
  Clippy now warns about code that calls `.map(...).flatten()` on an
  iterator and suggests `.flat_map(...)` instead:
  This branch fixes one place where we do that. CI should be happy again.
diff: @@ -1092,12 +1092,7 @@ where
  ctx.format_fields(writer.by_ref(), event)?;
  - for span in ctx
  - .event_scope()
  - .into_iter()
  - .map(Scope::from_root)
  - .flatten()
  - {
  + for span in ctx.event_scope().into_iter().flat_map(Scope::from_root) {
  let exts = span.extensions();
  if let Some(fields) = exts.get::<FormattedFields<N>>() {
  if !fields.is_empty() {
  