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/OpcUa/OpcUaPublishActor.cs

@@ -3,6 +3,7 @@ using Akka.Cluster.Tools.PublishSubscribe;
 using Akka.Event;
 using Microsoft.EntityFrameworkCore;
 using ZB.MOM.WW.OtOpcUa.Commons.Messages.Redundancy;
+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.Configuration;
@@ -124,6 +125,7 @@ public sealed class OpcUaPublishActor : ReceiveActor
         {
             _sink.WriteValue(msg.NodeId, msg.Value, msg.Quality, msg.TimestampUtc);
             Interlocked.Increment(ref _writes);
+            OtOpcUaTelemetry.OpcUaSinkWrite.Add(1, new KeyValuePair<string, object?>("kind", "value"));
         }
         catch (Exception ex)
         {
@@ -137,6 +139,7 @@ public sealed class OpcUaPublishActor : ReceiveActor
         {
             _sink.WriteAlarmState(msg.AlarmNodeId, msg.Active, msg.Acknowledged, msg.TimestampUtc);
             Interlocked.Increment(ref _writes);
+            OtOpcUaTelemetry.OpcUaSinkWrite.Add(1, new KeyValuePair<string, object?>("kind", "alarm"));
         }
         catch (Exception ex)
         {
@@ -146,12 +149,19 @@ public sealed class OpcUaPublishActor : ReceiveActor
 
     private void HandleRebuild(RebuildAddressSpace msg)
     {
+        using var span = OtOpcUaTelemetry.StartAddressSpaceRebuildSpan();
+        span?.SetTag("otopcua.correlation_id", msg.Correlation.ToString());
+
         // Two modes: when dbFactory + applier are wired, do a real diff-and-apply pass against
         // the latest deployment artifact. Without them, fall back to a raw sink rebuild — the
         // F10b/dev path before the integration completes.
         if (_dbFactory is null || _applier is null)
         {
-            try { _sink.RebuildAddressSpace(); }
+            try
+            {
+                _sink.RebuildAddressSpace();
+                OtOpcUaTelemetry.OpcUaSinkWrite.Add(1, new KeyValuePair<string, object?>("kind", "rebuild"));
+            }
             catch (Exception ex)
             {
                 _log.Error(ex, "OpcUaPublish: sink.RebuildAddressSpace threw (correlation={Correlation})",
@@ -175,6 +185,7 @@ public sealed class OpcUaPublishActor : ReceiveActor
 
             var outcome = _applier.Apply(plan);
             _lastApplied = composition;
+            OtOpcUaTelemetry.OpcUaSinkWrite.Add(1, new KeyValuePair<string, object?>("kind", "rebuild"));
             _log.Info("OpcUaPublish: applied rebuild (correlation={Correlation}, added={Added}, removed={Removed}, changed={Changed}, rebuild={Rebuild})",
                 msg.Correlation, outcome.AddedNodes, outcome.RemovedNodes, outcome.ChangedNodes, outcome.RebuildCalled);
         }
@@ -211,6 +222,8 @@ public sealed class OpcUaPublishActor : ReceiveActor
         try
         {
             _serviceLevel.Publish(msg.ServiceLevel);
+            OtOpcUaTelemetry.ServiceLevelChange.Add(1,
+                new KeyValuePair<string, object?>("level", msg.ServiceLevel));
             _log.Debug("OpcUaPublish: ServiceLevel={Level}", msg.ServiceLevel);
         }
         catch (Exception ex)