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/Metrics/AlarmProviderSwitchReason.cs

@@ -0,0 +1,20 @@
+namespace ZB.MOM.WW.MxGateway.Server.Metrics;
+
+/// <summary>
+///     Bounded classification of an alarm-provider switch, used as the low-cardinality
+///     <c>reason</c> tag on the <c>mxgateway.alarms.provider_switches</c> counter. The
+///     worker supplies a free-text reason (e.g. <c>"primary PollOnce failed"</c>) that
+///     stays in the structured log; only this bounded value reaches the metric tag so the
+///     time series cannot fan out on operation-specific text.
+/// </summary>
+public enum AlarmProviderSwitchReason
+{
+    /// <summary>The switch direction could not be classified.</summary>
+    Unknown = 0,
+
+    /// <summary>Switched from the primary (alarmmgr) provider to the subtag standby — degraded.</summary>
+    Failover = 1,
+
+    /// <summary>Switched back from the subtag standby to the primary (alarmmgr) provider — recovered.</summary>
+    Failback = 2,
+}