subscriber: add missing Filter::on_record for to EnvFilter (tokio…

…-rs#2058)

Depends on tokio-rs#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

@@ -629,6 +629,18 @@ impl EnvFilter {
         }
     }
 
+    /// 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)