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/subscriber_filters/combinator.rs

@@ -3,7 +3,7 @@ use crate::subscribe::{Context, Filter};
 use std::{cmp, fmt, marker::PhantomData};
 use tracing_core::{
     collect::Interest,
-    span::{Attributes, Id},
+    span::{Attributes, Id, Record},
     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/subscriber_filters/mod.rs

@@ -587,7 +587,8 @@ where
 
     fn on_record(&self, span: &span::Id, values: &span::Record<'_>, cx: Context<'_, C>) {
         if let Some(cx) = cx.if_enabled_for(span, self.id()) {
-            self.subscriber.on_record(span, values, cx)
+            self.filter.on_record(span, values, cx.clone());
+            self.subscriber.on_record(span, values, cx);
         }
     }
 

tracing-subscriber/src/subscribe/mod.rs

@@ -1242,6 +1242,16 @@ pub trait Filter<S> {
         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