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

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus/ModbusDriver.cs

@@ -3,6 +3,7 @@ using System.Collections.Concurrent;
 using System.Text.Json;
 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.Modbus;
@@ -187,7 +188,12 @@ public sealed class ModbusDriver
             {
                 if (_cycleBatchItems.TryRemove(handle, out var published) && published.Count > 0)
                     OnDataChangeBatch?.Invoke(this, new DataChangeBatchEventArgs(handle, published));
-            });
+            },
+            // 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.Modbus),
+                new KeyValuePair<string, object?>("driver.instance", driverInstanceId)));
     }
 
     /// <summary>Upper bound on the poll-loop failure backoff — adopts the S7-proven 30 s cap fleet-wide (05/STAB-8).</summary>