feat: add per-account message/byte stats with Interlocked counters

src/NATS.Server/Auth/Account.cs

@@ -48,5 +48,28 @@ public sealed class Account : IDisposable
         Interlocked.Decrement(ref _subscriptionCount);
     }
 
+    // Per-account message/byte stats
+    private long _inMsgs;
+    private long _outMsgs;
+    private long _inBytes;
+    private long _outBytes;
+
+    public long InMsgs => Interlocked.Read(ref _inMsgs);
+    public long OutMsgs => Interlocked.Read(ref _outMsgs);
+    public long InBytes => Interlocked.Read(ref _inBytes);
+    public long OutBytes => Interlocked.Read(ref _outBytes);
+
+    public void IncrementInbound(long msgs, long bytes)
+    {
+        Interlocked.Add(ref _inMsgs, msgs);
+        Interlocked.Add(ref _inBytes, bytes);
+    }
+
+    public void IncrementOutbound(long msgs, long bytes)
+    {
+        Interlocked.Add(ref _outMsgs, msgs);
+        Interlocked.Add(ref _outBytes, bytes);
+    }
+
     public void Dispose() => SubList.Dispose();
 }

tests/NATS.Server.Tests/AccountStatsTests.cs

@@ -0,0 +1,48 @@
+using NATS.Server.Auth;
+
+namespace NATS.Server.Tests;
+
+public class AccountStatsTests
+{
+    [Fact]
+    public void Account_tracks_inbound_stats()
+    {
+        var account = new Account("test");
+        account.IncrementInbound(1, 100);
+        account.IncrementInbound(1, 200);
+        account.InMsgs.ShouldBe(2);
+        account.InBytes.ShouldBe(300);
+    }
+
+    [Fact]
+    public void Account_tracks_outbound_stats()
+    {
+        var account = new Account("test");
+        account.IncrementOutbound(1, 50);
+        account.IncrementOutbound(1, 75);
+        account.OutMsgs.ShouldBe(2);
+        account.OutBytes.ShouldBe(125);
+    }
+
+    [Fact]
+    public void Account_stats_start_at_zero()
+    {
+        var account = new Account("test");
+        account.InMsgs.ShouldBe(0);
+        account.OutMsgs.ShouldBe(0);
+        account.InBytes.ShouldBe(0);
+        account.OutBytes.ShouldBe(0);
+    }
+
+    [Fact]
+    public void Account_stats_are_independent()
+    {
+        var account = new Account("test");
+        account.IncrementInbound(5, 500);
+        account.IncrementOutbound(3, 300);
+        account.InMsgs.ShouldBe(5);
+        account.OutMsgs.ShouldBe(3);
+        account.InBytes.ShouldBe(500);
+        account.OutBytes.ShouldBe(300);
+    }
+}