perf: const-ify some logging thread-local variables (#3513)

Author: morrisonlevi

components-rs/log.rs

@@ -38,8 +38,10 @@ pub static mut ddog_log_callback: Option<extern "C" fn(CharSlice)> = None;
 
 // Avoid RefCell for performance
 std::thread_local! {
-    static LOGGED_MSGS: RefCell<BTreeSet<String>> = RefCell::default();
-    static TRACING_GUARDS: RefCell<Option<tracing_core::dispatcher::DefaultGuard>> = RefCell::default();
+    static LOGGED_MSGS: RefCell<BTreeSet<String>> = const { RefCell::new(BTreeSet::new()) };
+    static TRACING_GUARDS: RefCell<Option<tracing_core::dispatcher::DefaultGuard>> = const { RefCell::new(None) };
+
+    // todo: MSRV 1.85+ make this const with HashMap::with_hasher
     static COUNTERS: RefCell<HashMap<Level, u32>> = RefCell::default();
 }