Fix reliability findings

src/MxGateway.Server/Metrics/GatewayMetrics.cs

@@ -101,6 +101,17 @@ public sealed class GatewayMetrics : IDisposable
         _sessionsClosedCounter.Add(1);
     }
 
+    public void SessionRemoved()
+    {
+        lock (_syncRoot)
+        {
+            if (_openSessions > 0)
+            {
+                _openSessions--;
+            }
+        }
+    }
+
     public void WorkerStarted(TimeSpan startupDuration)
     {
         lock (_syncRoot)