perf(gateway): trim event/command hot-path allocations (GWC-06/07/15, IPC-05)

Behavior-preserving allocation cuts on the per-event/per-command path:
- GWC-06: StreamEvents send-timing uses Stopwatch.GetTimestamp() +
  GetElapsedTime() instead of a per-event Stopwatch allocation (same measured
  span).
- GWC-07/IPC-05 (event): MapEvent transfers ownership of the inner MxEvent
  instead of .Clone()-ing it. Safe: WorkerEvent is parsed fresh per pipe frame
  with the distributor pump as its single consumer (GWC-01), MapEvent runs once
  before fan-out, and every downstream consumer (subscribers, replay ring)
  only READS the event (WorkerSequence is stamped upstream; verified no
  post-mapping mutation). Comment documents the invariant + restore-clone
  caveat if a second consumer is added.
- IPC-05 (command): CreateCommandEnvelope no longer re-clones; MapCommand
  already isolated the graph from the caller-owned gRPC message.
- GWC-15: grpc_stream_queue.depth converts from a per-event push counter to an
  ObservableGauge summing registered channel sources at scrape time only
  (name/semantics unchanged); removes all per-event .Count/lock work.

Kept every load-bearing isolation clone (MapCommand, Invoke, bulk filters,
MapCommandReply). Server build clean (0 warnings); EventStream/Metrics/
Distributor/Mapper tests 62/62 (incl. formerly-flaky queue-depth tests, now
green under the lazy gauge, + 2 new MapEvent ownership tests). Docs: Metrics.md,
Grpc.md.

Claude-Session: https://claude.ai/code/session_01DMXXvNuPekkkrTEyPNxEkW

src/ZB.MOM.WW.MxGateway.Server/Grpc/EventStreamService.cs

@@ -116,11 +116,23 @@ public sealed class EventStreamService(
                 options.Value.Sessions.MaxEventSubscribersPerSession);
         }
 
-        int streamQueueDepth = 0;
         IAsyncEnumerator<MxEvent> reader = subscriber.Reader
             .ReadAllAsync(cancellationToken)
             .GetAsyncEnumerator(cancellationToken);
 
+        // GWC-15: register this subscriber's channel as a live backlog source instead of
+        // reconciling the queue-depth gauge on every event. The gauge previously read the
+        // bounded channel's Count (which takes the channel's internal lock) and adjusted the
+        // metric under its own lock on every streamed event. Now the metric reads Count only
+        // when it is scraped (ObservableGauge callback) or projected (GetSnapshot), summing the
+        // live backlog across every registered subscriber — the same "buffered, not yet
+        // delivered" aggregate the per-event push reported, but with no per-event lock traffic.
+        // Disposing the registration in the finally removes this subscriber's contribution, so
+        // the gauge returns to the other subscribers' backlog (zero when none remain) on
+        // disconnect. CanCount guards a channel that ever cannot report Count (contributes 0).
+        IDisposable backlogRegistration = metrics.RegisterEventStreamBacklogSource(
+            () => subscriber.Reader.CanCount ? subscriber.Reader.Count : 0);
+
         try
         {
             // Emit order for a resume: the ReplayGap sentinel FIRST (only when events were
@@ -179,32 +191,21 @@ public sealed class EventStreamService(
                     continue;
                 }
 
-                // Queue-depth gauge tracks events the pump has fanned into this subscriber's
-                // channel but the client has not yet consumed — the same "buffered, not yet
-                // delivered" quantity the original per-RPC channel reported. The bounded
-                // subscriber channel supports counting, so reconcile the gauge to the current
-                // backlog; falling back to a no-op delta if a channel ever cannot count.
-                int backlog = subscriber.Reader.CanCount ? subscriber.Reader.Count : streamQueueDepth;
-                int delta = backlog - streamQueueDepth;
-                if (delta != 0)
-                {
-                    streamQueueDepth = backlog;
-                    metrics.AdjustGrpcEventStreamQueueDepth(delta);
-                }
-
+                // The queue-depth gauge is maintained lazily via the backlog registration above
+                // (GWC-15): the metric reads this subscriber's channel Count only when scraped,
+                // so there is no per-event gauge bookkeeping on this hot path.
                 yield return mxEvent;
             }
         }
         finally
         {
             await reader.DisposeAsync().ConfigureAwait(false);
-            subscriber.Dispose();
 
-            if (streamQueueDepth != 0)
-            {
-                metrics.AdjustGrpcEventStreamQueueDepth(-streamQueueDepth);
-                streamQueueDepth = 0;
-            }
+            // Remove this subscriber's live backlog contribution before disposing the lease so
+            // the gauge stops counting a channel that is about to be completed; after this the
+            // gauge reflects only the remaining subscribers (zero when none remain).
+            backlogRegistration.Dispose();
+            subscriber.Dispose();
 
             metrics.StreamDisconnected("Detached");
         }

src/ZB.MOM.WW.MxGateway.Server/Grpc/MxAccessGatewayService.cs

@@ -152,9 +152,15 @@ public sealed class MxAccessGatewayService(
                 .WithCancellation(context.CancellationToken)
                 .ConfigureAwait(false))
             {
-                Stopwatch stopwatch = Stopwatch.StartNew();
+                // GWC-06: measure send latency with the allocation-free timestamp API rather
+                // than allocating a Stopwatch object per event per subscriber on the highest-
+                // volume gateway path. Stopwatch.GetTimestamp/GetElapsedTime measure the exact
+                // same wall-clock span the former Stopwatch.StartNew()/.Elapsed did.
+                long sendStartTimestamp = Stopwatch.GetTimestamp();
                 await responseStream.WriteAsync(publicEvent).ConfigureAwait(false);
-                metrics.RecordEventStreamSend(publicEvent.Family.ToString(), stopwatch.Elapsed);
+                metrics.RecordEventStreamSend(
+                    publicEvent.Family.ToString(),
+                    Stopwatch.GetElapsedTime(sendStartTimestamp));
             }
         }
         catch (Exception exception) when (exception is not RpcException)

src/ZB.MOM.WW.MxGateway.Server/Grpc/MxAccessGrpcMapper.cs

@@ -65,7 +65,16 @@ public sealed class MxAccessGrpcMapper
     {
         ArgumentNullException.ThrowIfNull(workerEvent);
 
-        return workerEvent.Event?.Clone() ?? new MxEvent
+        // GWC-07 / IPC-05: ownership transfer, not a deep clone. The enclosing WorkerEvent is
+        // parsed fresh from a single pipe frame in WorkerClient's read loop and is discarded
+        // immediately after this mapping — the SessionEventDistributor pump is its single
+        // consumer (GWC-01 claims the worker event channel as single-reader), so nothing else
+        // aliases or mutates workerEvent.Event. We therefore move the inner MxEvent into the
+        // outbound graph instead of cloning it. Downstream the pump fans this one MxEvent to
+        // every subscriber and retains it in the replay ring, but that sharing is READ-ONLY
+        // (subscribers only yield/filter it), so a single shared instance is safe. If a second
+        // consumer of WorkerEvent is ever added, restore a .Clone() here to re-isolate.
+        return workerEvent.Event ?? new MxEvent
         {
             Family = MxEventFamily.Unspecified,
             RawStatus = "Worker event did not contain a public event payload.",

src/ZB.MOM.WW.MxGateway.Server/Metrics/GatewayMetrics.cs

@@ -32,10 +32,17 @@ public sealed class GatewayMetrics : IDisposable
     private readonly ConcurrentDictionary<string, long> _eventsBySession = new(StringComparer.Ordinal);
     private readonly Dictionary<string, long> _retryAttemptsByArea = new(StringComparer.OrdinalIgnoreCase);
 
+    // GWC-15: live backlog sources for the gRPC event-stream queue-depth gauge. Each active
+    // StreamEvents subscriber registers a delegate returning its channel's current backlog; the
+    // gauge sums them ONLY when scraped or projected (GetGrpcEventStreamQueueDepth), so nothing is
+    // read on the per-event streaming hot path. Concurrent because subscribers register/unregister
+    // from arbitrary gRPC threads while the gauge callback / GetSnapshot enumerate.
+    private readonly ConcurrentDictionary<long, Func<int>> _eventStreamBacklogSources = new();
+    private long _nextEventStreamBacklogSourceId;
+
     private int _openSessions;
     private int _workersRunning;
     private int _workerEventQueueDepth;
-    private int _grpcEventStreamQueueDepth;
     private int _alarmProviderMode;
     private long _sessionsOpened;
     private long _sessionsClosed;
@@ -292,15 +299,24 @@ public sealed class GatewayMetrics : IDisposable
     }
 
     /// <summary>
-    /// Adjusts the gRPC event stream queue depth by the given delta.
+    ///     Registers a live backlog source for the gRPC event-stream queue-depth gauge and
+    ///     returns a handle that removes it when disposed. The gauge sums every registered
+    ///     source's current value on demand (when scraped or via <see cref="GetSnapshot"/>),
+    ///     rather than tracking a pushed running total, so the streaming hot path does no
+    ///     per-event gauge bookkeeping (GWC-15).
     /// </summary>
-    /// <param name="delta">Amount to adjust the queue depth by.</param>
-    public void AdjustGrpcEventStreamQueueDepth(int delta)
+    /// <param name="backlog">
+    ///     Returns this subscriber's current channel backlog. Invoked only at collection time;
+    ///     must be cheap and non-blocking (a bounded channel's <c>Count</c>). Negative returns
+    ///     are clamped to zero when summed.
+    /// </param>
+    /// <returns>A handle whose disposal unregisters the source. Safe to dispose more than once.</returns>
+    public IDisposable RegisterEventStreamBacklogSource(Func<int> backlog)
     {
-        lock (_syncRoot)
-        {
-            _grpcEventStreamQueueDepth = Math.Max(0, _grpcEventStreamQueueDepth + delta);
-        }
+        ArgumentNullException.ThrowIfNull(backlog);
+        long id = Interlocked.Increment(ref _nextEventStreamBacklogSourceId);
+        _eventStreamBacklogSources[id] = backlog;
+        return new EventStreamBacklogRegistration(this, id);
     }
 
     /// <summary>
@@ -429,13 +445,16 @@ public sealed class GatewayMetrics : IDisposable
     /// <returns>The current metrics snapshot.</returns>
     public GatewayMetricsSnapshot GetSnapshot()
     {
+        // Compute the live gRPC stream backlog outside _syncRoot: the sources are the subscriber
+        // channels' Count (their own locks) and must not run under this lock. GWC-15.
+        int grpcEventStreamQueueDepth = GetGrpcEventStreamQueueDepth();
         lock (_syncRoot)
         {
             return new GatewayMetricsSnapshot(
                 OpenSessions: _openSessions,
                 WorkersRunning: _workersRunning,
                 WorkerEventQueueDepth: _workerEventQueueDepth,
-                GrpcEventStreamQueueDepth: _grpcEventStreamQueueDepth,
+                GrpcEventStreamQueueDepth: grpcEventStreamQueueDepth,
                 SessionsOpened: _sessionsOpened,
                 SessionsClosed: _sessionsClosed,
                 CommandsStarted: _commandsStarted,
@@ -495,12 +514,28 @@ public sealed class GatewayMetrics : IDisposable
         }
     }
 
+    // Sums the live backlog across every registered event-stream subscriber. Runs at collection
+    // time (ObservableGauge scrape) or when GetSnapshot projects the value — never on the
+    // per-event path. Enumerating ConcurrentDictionary.Values never throws on concurrent
+    // register/unregister; a source removed mid-enumeration simply drops from this sample.
     private int GetGrpcEventStreamQueueDepth()
     {
-        lock (_syncRoot)
+        int total = 0;
+        foreach (Func<int> source in _eventStreamBacklogSources.Values)
         {
-            return _grpcEventStreamQueueDepth;
+            int value = source();
+            if (value > 0)
+            {
+                total += value;
+            }
         }
+
+        return total;
+    }
+
+    private void UnregisterEventStreamBacklogSource(long id)
+    {
+        _eventStreamBacklogSources.TryRemove(id, out _);
     }
 
     private int GetAlarmProviderMode()
@@ -521,4 +556,19 @@ public sealed class GatewayMetrics : IDisposable
     {
         values.AddOrUpdate(key, 1, static (_, currentValue) => currentValue + 1);
     }
+
+    // Handle returned by RegisterEventStreamBacklogSource. Disposal (once) removes the source
+    // from the gauge's live sum. Idempotent so a double dispose from a stream teardown is safe.
+    private sealed class EventStreamBacklogRegistration(GatewayMetrics metrics, long id) : IDisposable
+    {
+        private int _disposed;
+
+        public void Dispose()
+        {
+            if (Interlocked.Exchange(ref _disposed, 1) == 0)
+            {
+                metrics.UnregisterEventStreamBacklogSource(id);
+            }
+        }
+    }
 }

src/ZB.MOM.WW.MxGateway.Server/Workers/WorkerClient.cs

@@ -997,9 +997,14 @@ public sealed class WorkerClient : IWorkerClient
         string correlationId,
         WorkerCommand command)
     {
+        // IPC-05: no second clone. MxAccessGrpcMapper.MapCommand already deep-cloned the command
+        // out of the caller-owned gRPC MxCommandRequest, so this WorkerCommand is a fresh graph
+        // owned by the invoke pipeline and referenced by no other consumer. The envelope is built
+        // and owned entirely inside WorkerClient and the command is not touched again after this
+        // point, so transferring it into the envelope introduces no aliasing hazard.
         return CreateEnvelope(
             correlationId,
-            envelope => envelope.WorkerCommand = command.Clone());
+            envelope => envelope.WorkerCommand = command);
     }
 
     /// <summary>Creates shutdown envelope.</summary>

src/ZB.MOM.WW.MxGateway.Tests/Gateway/Grpc/MxAccessGrpcMapperTests.cs

@@ -76,4 +76,35 @@ public sealed class MxAccessGrpcMapperTests
 
         Assert.Equal(ProtocolStatusCode.ProtocolViolation, publicReply.ProtocolStatus.Code);
     }
+
+    /// <summary>
+    ///     Verifies MapEvent transfers ownership of the inner MxEvent (GWC-07 / IPC-05): the
+    ///     returned reference is the same instance carried by the WorkerEvent, not a clone. The
+    ///     WorkerEvent is discarded after mapping and the distributor pump is its single consumer,
+    ///     so moving the inner event out is safe and avoids a per-event deep copy.
+    /// </summary>
+    [Fact]
+    public void MapEvent_TransfersOwnershipOfInnerEventWithoutCloning()
+    {
+        MxEvent innerEvent = new()
+        {
+            Family = MxEventFamily.OnDataChange,
+            RawStatus = "OK",
+        };
+        WorkerEvent workerEvent = new() { Event = innerEvent };
+
+        MxEvent mapped = new MxAccessGrpcMapper().MapEvent(workerEvent);
+
+        Assert.Same(innerEvent, mapped);
+    }
+
+    /// <summary>Verifies that a worker event with no public payload maps to the unspecified-family sentinel.</summary>
+    [Fact]
+    public void MapEvent_WhenEventMissing_ReturnsUnspecifiedFamilySentinel()
+    {
+        MxEvent mapped = new MxAccessGrpcMapper().MapEvent(new WorkerEvent());
+
+        Assert.Equal(MxEventFamily.Unspecified, mapped.Family);
+        Assert.Equal("Worker event did not contain a public event payload.", mapped.RawStatus);
+    }
 }

src/ZB.MOM.WW.MxGateway.Tests/Metrics/GatewayMetricsTests.cs

@@ -20,7 +20,9 @@ public sealed class GatewayMetricsTests
         metrics.EventReceived("session-1", "OnDataChange");
         metrics.EventReceived("session-1", "OnDataChange");
         metrics.SetWorkerEventQueueDepth(7);
-        metrics.AdjustGrpcEventStreamQueueDepth(3);
+        // GWC-15: the gRPC stream queue-depth gauge sums live backlog sources at collection time
+        // rather than tracking a pushed running total. Register a source reporting 3.
+        using IDisposable backlogSource = metrics.RegisterEventStreamBacklogSource(static () => 3);
         metrics.QueueOverflow("session-events");
         metrics.Fault("CommandTimeout");
         metrics.WorkerKilled("CommandTimeout");