subscriber: add missing Filter::on_record for to EnvFilter (#2058)

Depends on #2057

## Motivation

Currently, `EnvFilter`'s `Layer` implementation provides a
`Layer::on_record` method, but its `Filter` implementation is
missing the corresponding `Filter::on_record` implementation. This means
that when using `EnvFilter` as a per-layer filter, recording span
fields after the spans were created will not update the filter state.

## Solution

This commit factors out the `on_record` implementation for `Layer`
into an inherent method, and adds a new `Filter::on_record` method that
calls it as well.

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

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

@@ -588,6 +588,18 @@ impl EnvFilter {
         spans.remove(&id);
     }
 
+    /// Informs the filter that the span with the provided `id` recorded the
+    /// provided field `values`.
+    ///
+    /// This is equivalent to calling the [`Layer::on_record`] or
+    /// [`Filter::on_record`] methods on `EnvFilter`'s implementations of those
+    /// traits, but it does not require the trait to be in scope
+    pub fn on_record<S>(&self, id: &span::Id, values: &span::Record<'_>, _: Context<'_, S>) {
+        if let Some(span) = try_lock!(self.by_id.read()).get(id) {
+            span.record_update(values);
+        }
+    }
+
     fn cares_about_span(&self, span: &span::Id) -> bool {
         let spans = try_lock!(self.by_id.read(), else return false);
         spans.contains_key(span)
@@ -643,10 +655,9 @@ impl<S: Subscriber> Layer<S> for EnvFilter {
         self.on_new_span(attrs, id, ctx)
     }
 
-    fn on_record(&self, id: &span::Id, values: &span::Record<'_>, _: Context<'_, S>) {
-        if let Some(span) = try_lock!(self.by_id.read()).get(id) {
-            span.record_update(values);
-        }
+    #[inline]
+    fn on_record(&self, id: &span::Id, values: &span::Record<'_>, ctx: Context<'_, S>) {
+        self.on_record(id, values, ctx);
     }
 
     #[inline]
@@ -690,6 +701,11 @@ feature! {
             self.on_new_span(attrs, id, ctx)
         }
 
+        #[inline]
+        fn on_record(&self, id: &span::Id, values: &span::Record<'_>, ctx: Context<'_, S>) {
+            self.on_record(id, values, ctx);
+        }
+
         #[inline]
         fn on_enter(&self, id: &span::Id, ctx: Context<'_, S>) {
             self.on_enter(id, ctx);