subscriber: clear per-layer interest when short circuiting (#1569)

Currently, when evaluating `register_callsite` for a stack containing
per-layer filters, the intermediate `Interest` from combining the per
layer filters' `Interest`s is stored in the thread-local `FilterState`.
When all per-layer filters have been evaluated, we reach the `Registry`,
which clears the `FilterState` and bubbles the `Interest` back up.
However, when a _global_ filter in the stack returns `Interest::never`,
we short-circuit, and don't reach the `Registry`. This means the
`Interest` state is not cleared.

This branch adds code in `Layered` to ensure the per-layer filter state
is cleared when a global filter short circuits `Interest` evaluation.

This fixes #1563.

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

tracing-subscriber/src/subscribe/layered.rs

@@ -380,6 +380,12 @@ where
         // If the outer layer has disabled the callsite, return now so that
         // the inner layer/subscriber doesn't get its hopes up.
         if outer.is_never() {
+            // If per-layer filters are in use, and we are short-circuiting
+            // (rather than calling into the inner type), clear the current
+            // per-layer filter interest state.
+            #[cfg(feature = "registry")]
+            drop(filter::FilterState::take_interest());
+
             return outer;
         }