feat: complete final jetstream parity transport and runtime baselines

src/NATS.Server/Monitoring/GatewayzHandler.cs

@@ -0,0 +1,21 @@
+namespace NATS.Server.Monitoring;
+
+public sealed class GatewayzHandler
+{
+    private readonly NatsServer _server;
+
+    public GatewayzHandler(NatsServer server)
+    {
+        _server = server;
+    }
+
+    public object Build()
+    {
+        var gateways = _server.Stats.Gateways;
+        return new
+        {
+            gateways,
+            num_gateways = gateways,
+        };
+    }
+}