server(alarms): provider-mode gauge startup baseline; reconcile-lock comment; de-flake monitor test

src/ZB.MOM.WW.MxGateway.Server/Alarms/GatewayAlarmMonitor.cs

@@ -167,6 +167,10 @@ public sealed class GatewayAlarmMonitor : BackgroundService, IGatewayAlarmServic
             _providerSince = DateTimeOffset.UtcNow;
         }
 
+        // Align the observable gauge with the Alarmmgr baseline without recording
+        // a switch — the gauge was 0 (unknown) from construction until now.
+        _metrics.SetAlarmProviderMode(ModeToInt(AlarmProviderMode.Alarmmgr));
+
         GatewaySession session = await _sessionManager.OpenSessionAsync(
                 new SessionOpenRequest(BackendName, MonitorClientName, Guid.NewGuid().ToString("N"), CommandTimeout: null),
                 MonitorClientName,
@@ -405,6 +409,10 @@ public sealed class GatewayAlarmMonitor : BackgroundService, IGatewayAlarmServic
 
         try
         {
+            // Intentionally awaited OUTSIDE _sync: ReconcileAsync acquires _sync itself,
+            // so holding it across the await here would deadlock. Subscribers therefore
+            // see the ProviderStatus push (above) slightly before the cache is re-seeded
+            // by the reconcile — an accepted brief inconsistency.
             await ReconcileAsync(sessionId, cancellationToken).ConfigureAwait(false);
         }
         catch (OperationCanceledException)

src/ZB.MOM.WW.MxGateway.Server/Metrics/GatewayMetrics.cs

@@ -402,6 +402,12 @@ public sealed class GatewayMetrics : IDisposable
             new KeyValuePair<string, object?>("reason", reason));
     }
 
+    /// <summary>Sets the current alarm provider-mode gauge without recording a switch (e.g. startup baseline).</summary>
+    public void SetAlarmProviderMode(int mode)
+    {
+        lock (_syncRoot) { _alarmProviderMode = mode; }
+    }
+
     /// <summary>
     /// Returns a snapshot of all current metric values.
     /// </summary>