metrics(alarms): expose provider-switch count in snapshot, bound the reason tag

B1: add AlarmProviderSwitchCount to GatewayMetricsSnapshot so the switch total is
readable without scraping the OTEL counter.
B2: replace the free-text reason tag on mxgateway.alarms.provider_switches with a
bounded AlarmProviderSwitchReason enum (failover/failback/unknown); the human-readable
reason stays in the structured log.

src/ZB.MOM.WW.MxGateway.Tests/Metrics/GatewayMetricsTests.cs

@@ -111,12 +111,13 @@ public sealed class GatewayMetricsTests
             });
         listener.Start();
 
-        metrics.AlarmProviderSwitched(1, 2, "test");
+        metrics.AlarmProviderSwitched(1, 2, AlarmProviderSwitchReason.Failover);
 
         Assert.Equal(1, capturedValue);
         Assert.Equal("1", capturedFrom);
         Assert.Equal("2", capturedTo);
-        Assert.Equal("test", capturedReason);
+        Assert.Equal("failover", capturedReason);
+        Assert.Equal(1, metrics.GetSnapshot().AlarmProviderSwitchCount);
     }
 
     /// <summary>
@@ -150,7 +151,7 @@ public sealed class GatewayMetricsTests
             });
         listener.Start();
 
-        metrics.AlarmProviderSwitched(1, 2, "test");
+        metrics.AlarmProviderSwitched(1, 2, AlarmProviderSwitchReason.Failover);
         listener.RecordObservableInstruments();
 
         Assert.Equal(2, capturedMode);