subscriber: add Filter::on_{new_span, enter, exit, close} (#1973)

Closes #1955

Call those methods in the `Subscribe` methods for `Filtered` and
delegate them  in the filter combinators

## Motivation

Currently, if a `Filter` is going to implement dynamic filtering, it must
do this by querying the wrapped Subscriber for the current span context.
Since `Filter` only has callsite_enabled and enabled methods, a `Filter`
implementation cannot track the span context internally, the way a
subscriber can.

Unfortunately, this means that the current implementation of `EnvFilter`
can only implement `Subscribe` (and provide global filtering). It cannot
currently implement `Filter`, because it stores span context data
internally. See tokio-rs/tracing#1868 for
details.

## Proposal

We should add `on_new_span`, `on_enter`, `on_exit`, and `on_close` methods to
`Filter`. This would allow implementing `Filter`s (such as `EnvFilter`) that
internally track span states.

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

@@ -1,7 +1,11 @@
 //! Filter combinators
 use crate::subscribe::{Context, Filter};
 use std::{cmp, fmt, marker::PhantomData};
-use tracing_core::{collect::Interest, LevelFilter, Metadata};
+use tracing_core::{
+    collect::Interest,
+    span::{Attributes, Id},
+    LevelFilter, Metadata,
+};
 
 /// Combines two [`Filter`]s so that spans and events are enabled if and only if
 /// *both* filters return `true`.
@@ -132,6 +136,30 @@ where
         // If either hint is `None`, return `None`. Otherwise, return the most restrictive.
         cmp::min(self.a.max_level_hint(), self.b.max_level_hint())
     }
+
+    #[inline]
+    fn on_new_span(&self, attrs: &Attributes<'_>, id: &Id, ctx: Context<'_, S>) {
+        self.a.on_new_span(attrs, id, ctx.clone());
+        self.b.on_new_span(attrs, id, ctx)
+    }
+
+    #[inline]
+    fn on_enter(&self, id: &Id, ctx: Context<'_, S>) {
+        self.a.on_enter(id, ctx.clone());
+        self.b.on_enter(id, ctx);
+    }
+
+    #[inline]
+    fn on_exit(&self, id: &Id, ctx: Context<'_, S>) {
+        self.a.on_exit(id, ctx.clone());
+        self.b.on_exit(id, ctx);
+    }
+
+    #[inline]
+    fn on_close(&self, id: Id, ctx: Context<'_, S>) {
+        self.a.on_close(id.clone(), ctx.clone());
+        self.b.on_close(id, ctx);
+    }
 }
 
 impl<A, B, S> Clone for And<A, B, S>
@@ -289,6 +317,30 @@ where
         // If either hint is `None`, return `None`. Otherwise, return the less restrictive.
         Some(cmp::max(self.a.max_level_hint()?, self.b.max_level_hint()?))
     }
+
+    #[inline]
+    fn on_new_span(&self, attrs: &Attributes<'_>, id: &Id, ctx: Context<'_, S>) {
+        self.a.on_new_span(attrs, id, ctx.clone());
+        self.b.on_new_span(attrs, id, ctx)
+    }
+
+    #[inline]
+    fn on_enter(&self, id: &Id, ctx: Context<'_, S>) {
+        self.a.on_enter(id, ctx.clone());
+        self.b.on_enter(id, ctx);
+    }
+
+    #[inline]
+    fn on_exit(&self, id: &Id, ctx: Context<'_, S>) {
+        self.a.on_exit(id, ctx.clone());
+        self.b.on_exit(id, ctx);
+    }
+
+    #[inline]
+    fn on_close(&self, id: Id, ctx: Context<'_, S>) {
+        self.a.on_close(id.clone(), ctx.clone());
+        self.b.on_close(id, ctx);
+    }
 }
 
 impl<A, B, S> Clone for Or<A, B, S>
@@ -356,6 +408,26 @@ where
         // TODO(eliza): figure this out???
         None
     }
+
+    #[inline]
+    fn on_new_span(&self, attrs: &Attributes<'_>, id: &Id, ctx: Context<'_, S>) {
+        self.a.on_new_span(attrs, id, ctx);
+    }
+
+    #[inline]
+    fn on_enter(&self, id: &Id, ctx: Context<'_, S>) {
+        self.a.on_enter(id, ctx);
+    }
+
+    #[inline]
+    fn on_exit(&self, id: &Id, ctx: Context<'_, S>) {
+        self.a.on_exit(id, ctx);
+    }
+
+    #[inline]
+    fn on_close(&self, id: Id, ctx: Context<'_, S>) {
+        self.a.on_close(id, ctx);
+    }
 }
 
 impl<A, S> Clone for Not<A, S>

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

@@ -612,8 +612,9 @@ where
 
     fn on_new_span(&self, attrs: &span::Attributes<'_>, id: &span::Id, cx: Context<'_, C>) {
         self.did_enable(|| {
-            self.subscriber
-                .on_new_span(attrs, id, cx.with_filter(self.id()));
+            let cx = cx.with_filter(self.id());
+            self.filter.on_new_span(attrs, id, cx.clone());
+            self.subscriber.on_new_span(attrs, id, cx);
         })
     }
 
@@ -644,19 +645,22 @@ where
 
     fn on_enter(&self, id: &span::Id, cx: Context<'_, C>) {
         if let Some(cx) = cx.if_enabled_for(id, self.id()) {
-            self.subscriber.on_enter(id, cx)
+            self.filter.on_enter(id, cx.clone());
+            self.subscriber.on_enter(id, cx);
         }
     }
 
     fn on_exit(&self, id: &span::Id, cx: Context<'_, C>) {
         if let Some(cx) = cx.if_enabled_for(id, self.id()) {
-            self.subscriber.on_exit(id, cx)
+            self.filter.on_enter(id, cx.clone());
+            self.subscriber.on_exit(id, cx);
         }
     }
 
     fn on_close(&self, id: span::Id, cx: Context<'_, C>) {
         if let Some(cx) = cx.if_enabled_for(&id, self.id()) {
-            self.subscriber.on_close(id, cx)
+            self.filter.on_close(id.clone(), cx.clone());
+            self.subscriber.on_close(id, cx);
         }
     }
 

tracing-subscriber/src/subscribe/mod.rs

@@ -965,6 +965,40 @@ pub trait Filter<S> {
     fn max_level_hint(&self) -> Option<LevelFilter> {
         None
     }
+
+    /// Notifies this filter that a new span was constructed with the given
+    /// `Attributes` and `Id`.
+    ///
+    /// By default, this method does nothing. `Filter` implementations that
+    /// need to be notified when new spans are created can override this
+    /// method.
+    fn on_new_span(&self, attrs: &span::Attributes<'_>, id: &span::Id, ctx: Context<'_, S>) {
+        let _ = (attrs, id, ctx);
+    }
+
+    /// Notifies this filter that a span with the given ID was entered.
+    ///
+    /// By default, this method does nothing. `Filter` implementations that
+    /// need to be notified when a span is entered can override this method.
+    fn on_enter(&self, id: &span::Id, ctx: Context<'_, S>) {
+        let _ = (id, ctx);
+    }
+
+    /// Notifies this filter that a span with the given ID was exited.
+    ///
+    /// By default, this method does nothing. `Filter` implementations that
+    /// need to be notified when a span is exited can override this method.
+    fn on_exit(&self, id: &span::Id, ctx: Context<'_, S>) {
+        let _ = (id, ctx);
+    }
+
+    /// Notifies this filter that a span with the given ID has been closed.
+    ///
+    /// By default, this method does nothing. `Filter` implementations that
+    /// need to be notified when a span is closed can override this method.
+    fn on_close(&self, id: span::Id, ctx: Context<'_, S>) {
+        let _ = (id, ctx);
+    }
 }
 
 /// Extension trait adding a `with(Subscribe)` combinator to types implementing