subscriber: add Filter::on_record callback

Currently, `Filter` only has the `on_new_span`, `on_enter`, `on_exit`,
and `on_close` callbacks. This means it won't handle cases where a
span records a new field value that changes its filter state.

This branch adds the missing `on_record` callback.

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

tracing-subscriber/src/filter/layer_filters/combinator.rs

@@ -2,7 +2,7 @@
 use crate::layer::{Context, Filter};
 use std::{cmp, fmt, marker::PhantomData};
 use tracing_core::{
-    span::{Attributes, Id},
+    span::{Attributes, Id, Record},
     subscriber::Interest,
     LevelFilter, Metadata,
 };
@@ -143,6 +143,12 @@ where
         self.b.on_new_span(attrs, id, ctx)
     }
 
+    #[inline]
+    fn on_record(&self, id: &Id, values: &Record<'_>, ctx: Context<'_, S>) {
+        self.a.on_record(id, values, ctx.clone());
+        self.b.on_record(id, values, ctx);
+    }
+
     #[inline]
     fn on_enter(&self, id: &Id, ctx: Context<'_, S>) {
         self.a.on_enter(id, ctx.clone());
@@ -324,6 +330,12 @@ where
         self.b.on_new_span(attrs, id, ctx)
     }
 
+    #[inline]
+    fn on_record(&self, id: &Id, values: &Record<'_>, ctx: Context<'_, S>) {
+        self.a.on_record(id, values, ctx.clone());
+        self.b.on_record(id, values, ctx);
+    }
+
     #[inline]
     fn on_enter(&self, id: &Id, ctx: Context<'_, S>) {
         self.a.on_enter(id, ctx.clone());
@@ -414,6 +426,11 @@ where
         self.a.on_new_span(attrs, id, ctx);
     }
 
+    #[inline]
+    fn on_record(&self, id: &Id, values: &Record<'_>, ctx: Context<'_, S>) {
+        self.a.on_record(id, values, ctx.clone());
+    }
+
     #[inline]
     fn on_enter(&self, id: &Id, ctx: Context<'_, S>) {
         self.a.on_enter(id, ctx);

tracing-subscriber/src/filter/layer_filters/mod.rs

@@ -592,6 +592,7 @@ where
 
     fn on_record(&self, span: &span::Id, values: &span::Record<'_>, cx: Context<'_, S>) {
         if let Some(cx) = cx.if_enabled_for(span, self.id()) {
+            self.filter.on_record(span, values, cx.clone());
             self.layer.on_record(span, values, cx)
         }
     }

tracing-subscriber/src/layer/mod.rs

@@ -1065,6 +1065,17 @@ feature! {
             let _ = (attrs, id, ctx);
         }
 
+
+        /// Notifies this filter that a span with the given `Id` recorded the given
+        /// `values`.
+        ///
+        /// By default, this method does nothing. `Filter` implementations that
+        /// need to be notified when new spans are created can override this
+        /// method.
+        fn on_record(&self, id: &span::Id, values: &span::Record<'_>, ctx: Context<'_, S>) {
+            let _ = (id, values, ctx);
+        }
+
         /// Notifies this filter that a span with the given ID was entered.
         ///
         /// By default, this method does nothing. `Filter` implementations that