feat: add system request-reply monitoring services ($SYS.REQ.SERVER.*)

Register VARZ, HEALTHZ, SUBSZ, STATSZ, and IDZ request-reply handlers
on $SYS.REQ.SERVER.{id}.* subjects and $SYS.REQ.SERVER.PING.* wildcard
subjects via InitEventTracking. Also excludes the $SYS system account
from the /subz monitoring endpoint by default since its subscriptions
are internal infrastructure.

src/NATS.Server/Events/InternalEventSystem.cs

@@ -97,6 +97,45 @@ public sealed class InternalEventSystem : IAsyncDisposable
         }, ct);
     }
 
+    /// <summary>
+    /// Registers system request-reply monitoring services for this server.
+    /// Maps to Go's initEventTracking in events.go.
+    /// Sets up handlers for $SYS.REQ.SERVER.{id}.VARZ, HEALTHZ, SUBSZ, STATSZ, IDZ
+    /// and wildcard $SYS.REQ.SERVER.PING.* subjects.
+    /// </summary>
+    public void InitEventTracking(NatsServer server)
+    {
+        _server = server;
+        var serverId = server.ServerId;
+
+        // Server-specific monitoring services
+        RegisterService(serverId, "VARZ", server.HandleVarzRequest);
+        RegisterService(serverId, "HEALTHZ", server.HandleHealthzRequest);
+        RegisterService(serverId, "SUBSZ", server.HandleSubszRequest);
+        RegisterService(serverId, "STATSZ", server.HandleStatszRequest);
+        RegisterService(serverId, "IDZ", server.HandleIdzRequest);
+
+        // Wildcard ping services (all servers respond)
+        SysSubscribe(string.Format(EventSubjects.ServerPing, "VARZ"), WrapRequestHandler(server.HandleVarzRequest));
+        SysSubscribe(string.Format(EventSubjects.ServerPing, "HEALTHZ"), WrapRequestHandler(server.HandleHealthzRequest));
+        SysSubscribe(string.Format(EventSubjects.ServerPing, "IDZ"), WrapRequestHandler(server.HandleIdzRequest));
+        SysSubscribe(string.Format(EventSubjects.ServerPing, "STATSZ"), WrapRequestHandler(server.HandleStatszRequest));
+    }
+
+    private void RegisterService(string serverId, string name, Action<string, string?> handler)
+    {
+        var subject = string.Format(EventSubjects.ServerReq, serverId, name);
+        SysSubscribe(subject, WrapRequestHandler(handler));
+    }
+
+    private SystemMessageHandler WrapRequestHandler(Action<string, string?> handler)
+    {
+        return (sub, client, acc, subject, reply, hdr, msg) =>
+        {
+            handler(subject, reply);
+        };
+    }
+
     /// <summary>
     /// Publishes a $SYS.SERVER.{id}.STATSZ message with current server statistics.
     /// Maps to Go's sendStatsz in events.go.

src/NATS.Server/Monitoring/SubszHandler.cs

@@ -14,12 +14,16 @@ public sealed class SubszHandler(NatsServer server)
         var opts = ParseQueryParams(ctx);
         var now = DateTime.UtcNow;
 
-        // Collect subscriptions from all accounts (or filtered)
+        // Collect subscriptions from all accounts (or filtered).
+        // Exclude the $SYS system account unless explicitly requested — its internal
+        // subscriptions are infrastructure and not user-facing.
         var allSubs = new List<Subscription>();
         foreach (var account in server.GetAccounts())
         {
             if (!string.IsNullOrEmpty(opts.Account) && account.Name != opts.Account)
                 continue;
+            if (string.IsNullOrEmpty(opts.Account) && account.Name == "$SYS")
+                continue;
             allSubs.AddRange(account.SubList.GetAllSubscriptions());
         }
 
@@ -31,10 +35,10 @@ public sealed class SubszHandler(NatsServer server)
 
         var total = allSubs.Count;
         var numSubs = server.GetAccounts()
-            .Where(a => string.IsNullOrEmpty(opts.Account) || a.Name == opts.Account)
+            .Where(a => (string.IsNullOrEmpty(opts.Account) && a.Name != "$SYS") || a.Name == opts.Account)
             .Aggregate(0u, (sum, a) => sum + a.SubList.Count);
         var numCache = server.GetAccounts()
-            .Where(a => string.IsNullOrEmpty(opts.Account) || a.Name == opts.Account)
+            .Where(a => (string.IsNullOrEmpty(opts.Account) && a.Name != "$SYS") || a.Name == opts.Account)
             .Sum(a => a.SubList.CacheCount);
 
         SubDetail[] details = [];

src/NATS.Server/NatsServer.cs

@@ -398,6 +398,7 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
         _listeningStarted.TrySetResult();
 
         _eventSystem?.Start(this);
+        _eventSystem?.InitEventTracking(this);
 
         _logger.LogInformation("Listening for client connections on {Host}:{Port}", _options.Host, _options.Port);
 
@@ -724,6 +725,98 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
         _eventSystem?.Enqueue(new PublishMessage { Subject = subject, Body = msg });
     }
 
+    /// <summary>
+    /// Handles $SYS.REQ.SERVER.{id}.VARZ requests.
+    /// Returns core server information including stats counters.
+    /// </summary>
+    public void HandleVarzRequest(string subject, string? reply)
+    {
+        if (reply == null) return;
+        var varz = new
+        {
+            server_id = _serverInfo.ServerId,
+            server_name = _serverInfo.ServerName,
+            version = NatsProtocol.Version,
+            host = _options.Host,
+            port = _options.Port,
+            max_payload = _options.MaxPayload,
+            connections = ClientCount,
+            total_connections = Interlocked.Read(ref _stats.TotalConnections),
+            in_msgs = Interlocked.Read(ref _stats.InMsgs),
+            out_msgs = Interlocked.Read(ref _stats.OutMsgs),
+            in_bytes = Interlocked.Read(ref _stats.InBytes),
+            out_bytes = Interlocked.Read(ref _stats.OutBytes),
+        };
+        SendInternalMsg(reply, null, varz);
+    }
+
+    /// <summary>
+    /// Handles $SYS.REQ.SERVER.{id}.HEALTHZ requests.
+    /// Returns a simple health status response.
+    /// </summary>
+    public void HandleHealthzRequest(string subject, string? reply)
+    {
+        if (reply == null) return;
+        SendInternalMsg(reply, null, new { status = "ok" });
+    }
+
+    /// <summary>
+    /// Handles $SYS.REQ.SERVER.{id}.SUBSZ requests.
+    /// Returns the current subscription count.
+    /// </summary>
+    public void HandleSubszRequest(string subject, string? reply)
+    {
+        if (reply == null) return;
+        SendInternalMsg(reply, null, new { num_subscriptions = SubList.Count });
+    }
+
+    /// <summary>
+    /// Handles $SYS.REQ.SERVER.{id}.STATSZ requests.
+    /// Publishes current server statistics through the event system.
+    /// </summary>
+    public void HandleStatszRequest(string subject, string? reply)
+    {
+        if (reply == null) return;
+        var process = System.Diagnostics.Process.GetCurrentProcess();
+        var statsMsg = new Events.ServerStatsMsg
+        {
+            Server = BuildEventServerInfo(),
+            Stats = new Events.ServerStatsData
+            {
+                Start = StartTime,
+                Mem = process.WorkingSet64,
+                Cores = Environment.ProcessorCount,
+                Connections = ClientCount,
+                TotalConnections = Interlocked.Read(ref _stats.TotalConnections),
+                Subscriptions = SubList.Count,
+                InMsgs = Interlocked.Read(ref _stats.InMsgs),
+                OutMsgs = Interlocked.Read(ref _stats.OutMsgs),
+                InBytes = Interlocked.Read(ref _stats.InBytes),
+                OutBytes = Interlocked.Read(ref _stats.OutBytes),
+                SlowConsumers = Interlocked.Read(ref _stats.SlowConsumers),
+            },
+        };
+        SendInternalMsg(reply, null, statsMsg);
+    }
+
+    /// <summary>
+    /// Handles $SYS.REQ.SERVER.{id}.IDZ requests.
+    /// Returns basic server identity information.
+    /// </summary>
+    public void HandleIdzRequest(string subject, string? reply)
+    {
+        if (reply == null) return;
+        var idz = new
+        {
+            server_id = _serverInfo.ServerId,
+            server_name = _serverInfo.ServerName,
+            version = NatsProtocol.Version,
+            host = _options.Host,
+            port = _options.Port,
+        };
+        SendInternalMsg(reply, null, idz);
+    }
+
     /// <summary>
     /// Builds an EventServerInfo block for embedding in system event messages.
     /// Maps to Go's serverInfo() helper used in events.go advisory publishing.