feat(observability): F13d Prometheus + OpenTelemetry instrumentation

OtOpcUaTelemetry (Commons/Observability) centralizes the project's Meter
+ ActivitySource so all instrumentation points emit through a single
named surface. Counters cover the hot paths:

  otopcua.deploy.applied               (outcome=ack|reject)
  otopcua.deploy.apply.duration        (s, histogram)
  otopcua.driver.lifecycle             (event=spawn|spawn_stub|stop|fault)
  otopcua.virtualtag.eval              (outcome=ok|fail|skip)
  otopcua.scriptedalarm.transition     (state=activated|acknowledged|cleared)
  otopcua.opcua.sink.write             (kind=value|alarm|rebuild)
  otopcua.redundancy.service_level_change (level=byte)

Plus two ActivitySource spans:

  otopcua.deploy.apply                 wraps DriverHostActor.ApplyAndAck
  otopcua.opcua.address_space_rebuild  wraps OpcUaPublishActor.HandleRebuild

Instruments are no-op until a listener attaches, so tests + dev hosts
pay nothing for unread telemetry.

Host Program.cs gains AddOtOpcUaObservability() (binds the OtOpcUa Meter
+ ActivitySource to OpenTelemetry, attaches a Prometheus exporter) and
MapOtOpcUaMetrics() (mounts /metrics scrape endpoint). Driver-side
internals + ASP.NET request metrics deliberately stay off — the scrape
payload is scoped to OtOpcUa signals only.

Tests use MeterListener + ActivityListener to verify
VirtualTagActor.eval, OpcUaPublishActor.AttributeValueUpdate, and
RebuildAddressSpace actually emit on the central instruments. Runtime
suite is 72 / 72 green (+3).

Closes #105. Path A (F13b/c/d) complete; next batch options: #85 UNS
folder hierarchy in SDK, or F8b/F9b production engine bindings.

src/Server/ZB.MOM.WW.OtOpcUa.Runtime/Drivers/DriverInstanceActor.cs

@@ -1,5 +1,6 @@
 using Akka.Actor;
 using Akka.Event;
+using ZB.MOM.WW.OtOpcUa.Commons.Observability;
 using ZB.MOM.WW.OtOpcUa.Commons.OpcUa;
 using ZB.MOM.WW.OtOpcUa.Commons.Types;
 using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
@@ -82,6 +83,9 @@ public sealed class DriverInstanceActor : ReceiveActor, IWithTimers
         _driver = driver;
         _driverInstanceId = driver.DriverInstanceId;
         _reconnectInterval = reconnectInterval;
+        OtOpcUaTelemetry.DriverInstanceLifecycle.Add(1,
+            new KeyValuePair<string, object?>("event", startStubbed ? "spawn_stub" : "spawn"),
+            new KeyValuePair<string, object?>("driver_type", driver.DriverType));
         if (startStubbed)
         {
             Context.GetLogger().Info("[DEV-STUB] driver={Name} type={Type}",
@@ -314,5 +318,8 @@ public sealed class DriverInstanceActor : ReceiveActor, IWithTimers
         DetachSubscription();
         try { _driver.ShutdownAsync(CancellationToken.None).GetAwaiter().GetResult(); }
         catch (Exception ex) { _log.Warning(ex, "DriverInstance {Id}: ShutdownAsync threw on PostStop", _driverInstanceId); }
+        OtOpcUaTelemetry.DriverInstanceLifecycle.Add(1,
+            new KeyValuePair<string, object?>("event", "stop"),
+            new KeyValuePair<string, object?>("driver_type", _driver.DriverType));
     }
 }