subscriber: fix incorrect use of FormattedFields extension in fmt l…

…ayer

This fixes two bugs. One where the formatted fields were incorrectly
parameterized over Self instead of the formatter that generated the
formatted fields. The second bug is to check if another layer has
already inserted formatted fields and to avoid inserting if so, which
would cause a panic.

tracing-subscriber/src/fmt/fmt_layer.rs

@@ -395,22 +395,27 @@ 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();
-
-        let mut buf = String::new();
-        if self.fmt_fields.format_fields(&mut buf, attrs).is_ok() {
-            let fmt_fields = FormattedFields {
-                fields: buf,
-                _format_event: PhantomData::<fn(N)>,
-            };
-            extensions.insert(fmt_fields);
+        if let Some(FormattedFields { ref mut fields, .. }) =
+            extensions.get_mut::<FormattedFields<N>>()
+        {
+            let _ = self.fmt_fields.format_fields(fields, attrs);
+        } else {
+            let mut buf = String::new();
+            if self.fmt_fields.format_fields(&mut buf, attrs).is_ok() {
+                let fmt_fields = FormattedFields {
+                    fields: buf,
+                    _format_event: PhantomData::<fn(N)>,
+                };
+                extensions.insert(fmt_fields);
+            }
         }
     }
 
     fn on_record(&self, id: &Id, values: &Record<'_>, 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<Self>>()
+            extensions.get_mut::<FormattedFields<N>>()
         {
             let _ = self.fmt_fields.format_fields(fields, values);
         } else {