fix(r2-04): surface degraded address-space applies (Error log + otopcua.opcua.apply.failed)

src/Core/ZB.MOM.WW.OtOpcUa.Commons/Observability/OtOpcUaTelemetry.cs

@@ -58,6 +58,14 @@ public static class OtOpcUaTelemetry
         Meter.CreateCounter<long>("otopcua.opcua.sink.write", unit: "{write}",
             description: "Writes that landed in IOpcUaAddressSpaceSink (kind=value|alarm|rebuild).");
 
+    /// <summary>Address-space apply/materialise passes that swallowed at least one sink failure —
+    /// a failed rebuild (kind=rebuild) or per-node materialise failures (kind=nodes). A non-zero rate
+    /// means the running server holds a stale or partially-materialised address space despite a
+    /// reported-successful deploy (archreview 01/S-1).</summary>
+    public static readonly Counter<long> OpcUaApplyFailed =
+        Meter.CreateCounter<long>("otopcua.opcua.apply.failed", unit: "{apply}",
+            description: "Apply/materialise passes with swallowed sink failures (kind=rebuild|nodes).");
+
     public static readonly Counter<long> ServiceLevelChange =
         Meter.CreateCounter<long>("otopcua.redundancy.service_level_change", unit: "{change}",
             description: "OPC UA Server.ServiceLevel transitions emitted by the redundancy state.");