D1: surface AlarmProviderSwitchCount on dashboard metric list

src/ZB.MOM.WW.MxGateway.Server/Dashboard/DashboardSnapshotService.cs

@@ -195,6 +195,7 @@ public sealed class DashboardSnapshotService : IDashboardSnapshotService
             new("mxgateway.workers.exited", snapshot.WorkerExits),
             new("mxgateway.heartbeats.failed", snapshot.HeartbeatFailures),
             new("mxgateway.grpc.streams.disconnected", snapshot.StreamDisconnects),
+            new("mxgateway.alarms.provider_switches", snapshot.AlarmProviderSwitchCount),
         ];
 
         metrics.AddRange(snapshot.CommandFailuresByMethod

src/ZB.MOM.WW.MxGateway.Tests/Gateway/Dashboard/DashboardSnapshotServiceTests.cs

@@ -94,6 +94,24 @@ public sealed class DashboardSnapshotServiceTests
         Assert.Equal("worker pipe disconnected", fault.Message);
     }
 
+    /// <summary>
+    /// Verifies snapshot metrics include the cumulative alarm provider switch count.
+    /// </summary>
+    [Fact]
+    public void GetSnapshot_IncludesAlarmProviderSwitchCountMetric()
+    {
+        SessionRegistry registry = new();
+        using GatewayMetrics metrics = new();
+        metrics.AlarmProviderSwitched(1, 2, AlarmProviderSwitchReason.Failover);
+        DashboardSnapshotService service = CreateService(registry, metrics);
+
+        DashboardSnapshot snapshot = service.GetSnapshot();
+
+        Assert.Contains(
+            snapshot.Metrics,
+            metric => metric.Name == "mxgateway.alarms.provider_switches" && metric.Value == 1);
+    }
+
     /// <summary>
     /// Verifies snapshot redacts sensitive values from client identity, session name, and fault messages.
     /// </summary>