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

Closes #1955

Call those methods in the `Layer` 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 Layer
can.

Unfortunately, this means that the current implementation of `EnvFilter`
can only implement `Layer` (and provide global filtering). It cannot
currently implement `Filter`, because it stores span context data
internally. See https://github.com/tokio-rs/tracing/issues/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/layer_filters/combinator.rs

@@ -1,7 +1,11 @@
 //! Filter combinators
 use crate::layer::{Context, Filter};
 use std::{cmp, fmt, marker::PhantomData};
-use tracing_core::{subscriber::Interest, LevelFilter, Metadata};
+use tracing_core::{
+    span::{Attributes, Id},
+    subscriber::Interest,
+    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/layer_filters/mod.rs

@@ -579,7 +579,9 @@ where
 
     fn on_new_span(&self, attrs: &span::Attributes<'_>, id: &span::Id, cx: Context<'_, S>) {
         self.did_enable(|| {
-            self.layer.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.layer.on_new_span(attrs, id, cx);
         })
     }
 
@@ -610,19 +612,22 @@ where
 
     fn on_enter(&self, id: &span::Id, cx: Context<'_, S>) {
         if let Some(cx) = cx.if_enabled_for(id, self.id()) {
-            self.layer.on_enter(id, cx)
+            self.filter.on_enter(id, cx.clone());
+            self.layer.on_enter(id, cx);
         }
     }
 
     fn on_exit(&self, id: &span::Id, cx: Context<'_, S>) {
         if let Some(cx) = cx.if_enabled_for(id, self.id()) {
-            self.layer.on_exit(id, cx)
+            self.filter.on_enter(id, cx.clone());
+            self.layer.on_exit(id, cx);
         }
     }
 
     fn on_close(&self, id: span::Id, cx: Context<'_, S>) {
         if let Some(cx) = cx.if_enabled_for(&id, self.id()) {
-            self.layer.on_close(id, cx)
+            self.filter.on_close(id.clone(), cx.clone());
+            self.layer.on_close(id, cx);
         }
     }
 

tracing-subscriber/src/layer/mod.rs

@@ -457,7 +457,7 @@
 //!         .json()
 //!         .with_writer(Arc::new(file));
 //!     Some(json_log)
-//! } else {   
+//! } else {
 //!     None
 //! };
 //!
@@ -1054,6 +1054,40 @@ feature! {
         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);
+        }
     }
 }