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.Server/Alarms/GatewayAlarmMonitor.cs

@@ -399,7 +399,13 @@ public sealed class GatewayAlarmMonitor : BackgroundService, IGatewayAlarmServic
             BroadcastToAll(new AlarmFeedMessage { ProviderStatus = status });
         }
 
-        _metrics.AlarmProviderSwitched(fromModeInt, ModeToInt(toMode), reason);
+        AlarmProviderSwitchReason switchReason = toMode switch
+        {
+            AlarmProviderMode.Subtag => AlarmProviderSwitchReason.Failover,
+            AlarmProviderMode.Alarmmgr => AlarmProviderSwitchReason.Failback,
+            _ => AlarmProviderSwitchReason.Unknown,
+        };
+        _metrics.AlarmProviderSwitched(fromModeInt, ModeToInt(toMode), switchReason);
 
         _logger.LogInformation(
             "Alarm provider mode changed to {Mode} (degraded={Degraded}): {Reason}",