PR 6.1 — OpenTelemetry traces around gw calls

In-box ActivitySource ("ZB.MOM.WW.OtOpcUa.Driver.Galaxy") wrapped around
the three gw-facing seams via decorators:

- TracedGalaxySubscriber — galaxy.subscribe_bulk / galaxy.unsubscribe_bulk
  / galaxy.stream_events spans. Stream span covers the entire stream
  lifetime with a galaxy.event_count tag (per-event spans would dominate
  the trace volume at 50k tags / 1Hz; PR 6.2 owns per-event metrics).
- TracedGalaxyDataWriter — galaxy.write spans tagged with
  galaxy.tag_count, galaxy.secured_write_count (split between FreeAccess
  /Operate vs Tune/Configure/VerifiedWrite, computed only when a listener
  is recording so the hot path stays free), galaxy.success_count.
- TracedGalaxyHierarchySource — galaxy.get_hierarchy spans tagged with
  galaxy.object_count.

GalaxyDriver.BuildProductionRuntimeAsync wraps the production seams in
the decorators. The driver itself doesn't take an OpenTelemetry package
dependency — System.Diagnostics.ActivitySource is in-box; the host
process picks the listener.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/Browse/TracedGalaxyHierarchySource.cs

@@ -0,0 +1,30 @@
+using MxGateway.Contracts.Proto.Galaxy;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Runtime;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Browse;
+
+/// <summary>
+///     PR 6.1 — Decorator that emits one <see cref="System.Diagnostics.Activity"/> span
+///     per <c>GetHierarchy</c> RPC. <c>galaxy.object_count</c> on the span lets ops
+///     correlate slow Discover passes with Galaxy size without instrumenting the
+///     discoverer's translation step.
+/// </summary>
+internal sealed class TracedGalaxyHierarchySource(IGalaxyHierarchySource inner, string clientName) : IGalaxyHierarchySource
+{
+    public async Task<IReadOnlyList<GalaxyObject>> GetHierarchyAsync(CancellationToken cancellationToken)
+    {
+        using var activity = GalaxyTelemetry.ActivitySource.StartActivity("galaxy.get_hierarchy");
+        activity?.SetTag("galaxy.client", clientName);
+        try
+        {
+            var hierarchy = await inner.GetHierarchyAsync(cancellationToken).ConfigureAwait(false);
+            activity?.SetTag("galaxy.object_count", hierarchy.Count);
+            return hierarchy;
+        }
+        catch (Exception ex)
+        {
+            activity.RecordError(ex);
+            throw;
+        }
+    }
+}