[tracing-subscriber][tracing] improve duplciate tracing situation a bit

tracing-subscriber/src/fmt/fmt_layer.rs

@@ -797,6 +797,7 @@ impl<E: ?Sized> fmt::Display for FormattedFields<E> {
     }
 }
 
+// TODO: Evaluate removing Deref in future versions, as its a powerful tool and barely useful (consumers can just call `.fields` directly)
 impl<E: ?Sized> Deref for FormattedFields<E> {
     type Target = String;
     fn deref(&self) -> &Self::Target {
@@ -1631,8 +1632,7 @@ mod test {
             .with_timer(MockTime)
             .with_span_events(FmtSpan::ACTIVE);
 
-        let (reloadable_layer, reload_handle) =
-            crate::reload::Layer::new(inner_layer);
+        let (reloadable_layer, reload_handle) = crate::reload::Layer::new(inner_layer);
         let reload = reloadable_layer.with_subscriber(Registry::default());
 
         with_default(reload, || {

tracing-subscriber/src/fmt/format/mod.rs

@@ -969,7 +969,7 @@ where
 
                 let ext = span.extensions();
                 if let Some(fields) = &ext.get::<FormattedFields<N>>() {
-                    if !fields.is_empty() {
+                    if !fields.fields.is_empty() {
                         write!(writer, "{}{}{}", bold.paint("{"), fields, bold.paint("}"))?;
                     }
                 }
@@ -1146,7 +1146,7 @@ where
         {
             let exts = span.extensions();
             if let Some(fields) = exts.get::<FormattedFields<N>>() {
-                if !fields.is_empty() {
+                if !fields.fields.is_empty() {
                     write!(writer, " {}", dimmed.paint(&fields.fields))?;
                 }
             }

tracing-subscriber/src/fmt/format/pretty.rs

@@ -324,7 +324,7 @@ where
             let fields = &ext
                 .get::<FormattedFields<N>>()
                 .expect("Unable to find FormattedFields in extensions; this is a bug");
-            if !fields.is_empty() {
+            if !fields.fields.is_empty() {
                 write!(writer, " {} {}", dimmed.paint("with"), fields)?;
             }
             writer.write_char('\n')?;
@@ -346,7 +346,7 @@ impl<'writer> FormatFields<'writer> for Pretty {
         current: &'writer mut FormattedFields<Self>,
         fields: &span::Record<'_>,
     ) -> fmt::Result {
-        let empty = current.is_empty();
+        let empty = current.fields.is_empty();
         let writer = current.as_writer();
         let mut v = PrettyVisitor::new(writer, empty);
         fields.record(&mut v);

tracing/src/span.rs

@@ -1187,6 +1187,11 @@ impl Span {
     /// **Note**: To record several values in just one call, see the [`record_all!`](crate::record_all!) macro.
     /// </pre></div>
     ///
+    /// <pre class="ignore" style="white-space:normal;font:inherit;">
+    ///     <strong>Note</strong>: <a href="https://docs.rs/tracing-subscriber/latest/tracing_subscriber/fmt/struct.Subscriber.html"><code>FmtSubscriber</code></a> has a bug
+    ///     where it might duplicates span fields because of its implementation. See details at:  <a href="https://github.com/tokio-rs/tracing/issues/1372">Github</a>
+    /// </pre>
+    ///
     /// [`field::Empty`]: super::field::Empty
     /// [`Metadata`]: super::Metadata
     pub fn record<Q: field::AsField + ?Sized, V: field::Value>(