tracing: add index API for Field

Signed-off-by: hi-rustin <[email protected]>

tracing-core/src/field.rs

@@ -648,6 +648,11 @@ impl Field {
     pub fn name(&self) -> &'static str {
         self.fields.names[self.i]
     }
+
+    /// Returns the index of this field in its [`FieldSet`].
+    pub fn index(&self) -> usize {
+        self.i
+    }
 }
 
 impl fmt::Display for Field {