feat(drivers): per-driver poll-overrun meter on the shared OtOpcUa Meter; NoteOverrun + backoffCap doc notes

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.FOCAS/FocasDriver.cs

@@ -1,6 +1,7 @@
 using System.Collections.Concurrent;
 using Microsoft.Extensions.Logging;
 using Microsoft.Extensions.Logging.Abstractions;
+using ZB.MOM.WW.OtOpcUa.Commons.Observability;
 using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
 
 namespace ZB.MOM.WW.OtOpcUa.Driver.FOCAS;
@@ -121,7 +122,12 @@ public sealed class FocasDriver : IDriver, IReadable, IWritable, ITagDiscovery,
             onError: HandlePollError,
             backoffCap: PollBackoffCap,
             onBatch: (handle, items) =>
-                OnDataChangeBatch?.Invoke(this, new DataChangeBatchEventArgs(handle, items)));
+                OnDataChangeBatch?.Invoke(this, new DataChangeBatchEventArgs(handle, items)),
+            // The engine owns no Meter (Core.Abstractions takes no metrics dependency), so this
+            // closure IS the production consumer of its overrun signal.
+            onOverrun: _ => OtOpcUaTelemetry.PollOverruns.Add(1,
+                new KeyValuePair<string, object?>("driver.type", DriverTypeNames.FOCAS),
+                new KeyValuePair<string, object?>("driver.instance", driverInstanceId)));
     }
 
     /// <summary>True when the supplied DriverConfig JSON carries a real body. The bootstrapper always passes