fix: correct SerializeField definition and doc formatting (#3040)

Clippy in 1.80.0 alerted us to the fact that `SerializeField` was never
constructed (and due to its non-`pub` member, it can't be constructed
outside the `tracing-serde` crate where it's from).

This change fixes the definition to hold a reference to a `Field`, which
is what the other `Serialize*` types do. It also implements `AsSerde`
for this type and uses it inside the `SerializeFieldSet` type.

As a bonus, Clippy is now also happy that the type is constructed.

The example collector in the `tracing-serde` crate was also renamed from
`JsonSubscriber` to `JsonCollector`.

Some additional doc formatting issues in `tracing-subscriber` were fixed
so that list items that run to multiple lines are correctly indented.

tracing-serde/src/lib.rs

@@ -75,6 +75,12 @@
 //!     next_id: AtomicUsize, // you need to assign span IDs, so you need a counter
 //! }
 //!
+//! impl JsonSubscriber {
+//!     fn new() -> Self {
+//!         Self { next_id: 1.into() }
+//!     }
+//! }
+//!
 //! impl Subscriber for JsonSubscriber {
 //!
 //!     fn new_span(&self, attrs: &Attributes<'_>) -> Id {
@@ -97,7 +103,7 @@
 //!     }
 //!
 //!     // ...
-//!     # fn enabled(&self, _: &Metadata<'_>) -> bool { false }
+//!     # fn enabled(&self, _: &Metadata<'_>) -> bool { true }
 //!     # fn enter(&self, _: &Id) {}
 //!     # fn exit(&self, _: &Id) {}
 //!     # fn record(&self, _: &Id, _: &Record<'_>) {}
@@ -199,6 +205,18 @@ use tracing_core::{
 
 pub mod fields;
 
+#[derive(Debug)]
+pub struct SerializeField<'a>(&'a Field);
+
+impl<'a> Serialize for SerializeField<'a> {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        serializer.serialize_str(self.0.name())
+    }
+}
+
 #[derive(Debug)]
 pub struct SerializeFieldSet<'a>(&'a FieldSet);
 
@@ -209,7 +227,7 @@ impl<'a> Serialize for SerializeFieldSet<'a> {
     {
         let mut seq = serializer.serialize_seq(Some(self.0.len()))?;
         for element in self.0 {
-            seq.serialize_element(element.name())?;
+            seq.serialize_element(&SerializeField(&element))?;
         }
         seq.end()
     }
@@ -552,6 +570,14 @@ impl<'a> AsSerde<'a> for Level {
     }
 }
 
+impl<'a> AsSerde<'a> for Field {
+    type Serializable = SerializeField<'a>;
+
+    fn as_serde(&'a self) -> Self::Serializable {
+        SerializeField(self)
+    }
+}
+
 impl<'a> AsSerde<'a> for FieldSet {
     type Serializable = SerializeFieldSet<'a>;
 
@@ -572,6 +598,8 @@ impl<'a> self::sealed::Sealed for Record<'a> {}
 
 impl<'a> self::sealed::Sealed for Metadata<'a> {}
 
+impl self::sealed::Sealed for Field {}
+
 impl self::sealed::Sealed for FieldSet {}
 
 mod sealed {