subscriber: remove clone_span on enter (#3289)

## Motivation

Continuous profiling on a production workload shows that `Span::enter` is
a significant percent of the samples.

This is currently using `tracing-subscriber = "0.3.19"`.

Concrete numbers:
- 7.7% of samples were found within the stacktrace of a `Span::enter`.
- Of those samples, 22.8% were inside `clone_span`.
- Collectively, that's 1.8% of total CPU time spent in `clone_span` as a result of
  `Span::enter`.

## Solution

Remove `clone_span` from `enter`, remove `try_close` from `exit`.

This was originally included to protect against misuse of the `tracing-core` API.
However, there has not been so much use of this low level API as was originally
anticipated, and it is not worth the performance hit.

Author: conradludgate

tracing-subscriber/src/registry/sharded.rs

@@ -288,21 +288,15 @@ impl Subscriber for Registry {
     fn event(&self, _: &Event<'_>) {}
 
     fn enter(&self, id: &span::Id) {
-        if self
-            .current_spans
+        self.current_spans
             .get_or_default()
             .borrow_mut()
-            .push(id.clone())
-        {
-            self.clone_span(id);
-        }
+            .push(id.clone());
     }
 
     fn exit(&self, id: &span::Id) {
         if let Some(spans) = self.current_spans.get() {
-            if spans.borrow_mut().pop(id) {
-                self.try_close(id.clone());
-            }
+            spans.borrow_mut().pop(id);
         }
     }
 

tracing-subscriber/src/registry/stack.rs

@@ -18,10 +18,9 @@ pub(crate) struct SpanStack {
 
 impl SpanStack {
     #[inline]
-    pub(super) fn push(&mut self, id: Id) -> bool {
+    pub(super) fn push(&mut self, id: Id) {
         let duplicate = self.stack.iter().any(|i| i.id == id);
         self.stack.push(ContextId { id, duplicate });
-        !duplicate
     }
 
     #[inline]