tracer: reevaluate #[allow] attributes

Convert the still-needed #[allow] to #[expect] so it warns if the
lint stops firing.

Part of #8326.

Signed-off-by: Tushar Khatri <hello@tusharkhatri.in>

tracer/src/tracer.rs

@@ -3,7 +3,7 @@
 // SPDX-License-Identifier: Apache-2.0
 //
 
-#![allow(static_mut_refs)]
+#![expect(static_mut_refs)]
 
 use std::cell::OnceCell;
 use std::collections::HashMap;