feat(transactions): add single-node transaction support with session-aware reads, commits, aborts, and transaction metrics

rust/crates/rustdb-auth/src/lib.rs

@@ -150,6 +150,13 @@ impl AuthEngine {
         self.enabled
     }
 
+    pub fn user_count(&self) -> usize {
+        self.users
+            .read()
+            .unwrap_or_else(|poisoned| poisoned.into_inner())
+            .len()
+    }
+
     pub fn supported_mechanisms(&self, namespace_user: &str) -> Vec<String> {
         let Some((database, username)) = namespace_user.split_once('.') else {
             return Vec::new();