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

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

@@ -42,6 +42,19 @@ public static class OtOpcUaTelemetry
         Meter.CreateCounter<long>("otopcua.driver.lifecycle", unit: "{event}",
             description: "DriverInstanceActor lifecycle transitions (event=spawn|stop|fault).");
 
+    /// <summary>
+    /// Poll cycles that ran longer than their subscription's publishing interval, as counted by
+    /// <c>PollGroupEngine.OverrunCount</c>. <c>Core.Abstractions</c> deliberately carries no metrics
+    /// dependency, so each poll driver wires this counter to the engine's <c>onOverrun</c> callback;
+    /// this is the production consumer that surface was always missing. Tagged
+    /// <c>driver.type</c> (a <c>DriverTypeNames</c> constant) + <c>driver.instance</c>.
+    /// A non-zero and climbing rate means the device cannot sustain its configured poll rate — the
+    /// loop is already running flat out.
+    /// </summary>
+    public static readonly Counter<long> PollOverruns =
+        Meter.CreateCounter<long>("otopcua.driver.poll.overrun", unit: "{overrun}",
+            description: "Poll cycles that ran longer than the poll interval (slow success and slow failure both count — see PollGroupEngine.NoteOverrun).");
+
     // ---------------- VirtualTag / ScriptedAlarm engines ----------------
 
     public static readonly Counter<long> VirtualTagEval =