fix(trading_engine): tolerate AlreadyReg in metrics initialization test

Prometheus registry.register() returns AlreadyReg when another test
thread triggers the lazy_static counters first. Both Ok and AlreadyReg
are valid — only hard errors indicate a real problem.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

crates/trading_engine/src/types/metrics.rs

@@ -1311,7 +1311,18 @@ mod tests {
 
     #[test]
     fn test_metrics_initialization() {
-        assert!(initialize_metrics().is_ok());
+        // initialize_metrics() returns AlreadyReg if another test in this
+        // process already triggered the lazy_static counters. Both Ok and
+        // AlreadyReg are acceptable — only a hard error would be a bug.
+        let result = initialize_metrics();
+        assert!(
+            result.is_ok()
+                || result
+                    .as_ref()
+                    .err()
+                    .map_or(false, |e| format!("{e}").contains("already")),
+            "initialize_metrics failed with unexpected error: {result:?}"
+        );
     }
 
     #[test]