metrics: optional OpenMetrics/Prometheus endpoint (metrics_bind, off by default) — commands/messages/connects counters bumped inline via shared atomics, users/channels/servers/links gauges republished each tick; no event round-trip on the hot path

src/users.rs

@@ -439,6 +439,9 @@ impl Server {
         if let Some(u) = self.users.get_mut(&uid) {
             u.registered = true;
         }
+        self.metrics
+            .connects
+            .fetch_add(1, std::sync::atomic::Ordering::Relaxed);
         let nick = self
             .users
             .get(&uid)