subscriber: fall back to checking statics in enabled

Currently, the `tracing-subscriber` `EnvFilter` does not properly enable
events that originate from the `tracing-log` `LogTracer` adapter. This
is because those events are supposed to be filtered only by `enabled`,
and will not generate proper `register_callsite` metadata. `EnvFilter`'s
`enabled` method will assume that an event was not enabled statically,
since `enabled` wouldn't have been called if the callsite was given the
`Always` interest. However, this doesn't take `tracing-log` into
account.

I've changed `EnvFilter` to always check against static filters in
`enabled` if the dynamic filters don't enable a given metadata. This
fixes the filtering of `tracing-log` events. I've also added tests that
fail against the current master.

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

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

@@ -231,11 +231,15 @@ impl<S: Subscriber> Layer<S> for EnvFilter {
                     }
                 }
 
-                // TODO: other filters...
-
-                false
+                // Otherwise, fall back to checking if the callsite is
+                // statically enabled.
+                // TODO(eliza): we *might* want to check this only if the `log`
+                // feature is enabled, since if this is a `tracing` event with a
+                // real callsite, it would already have been statically enabled...
+                self.statics.enabled(metadata)
+            }).unwrap_or_else(|| {
+                self.statics.enabled(metadata)
             })
-            .unwrap_or(false)
     }
 
     fn new_span(&self, attrs: &span::Attributes<'_>, id: &span::Id, _: Context<'_, S>) {