feat(sessions): per-subscriber backpressure isolation in SessionEventDistributor

src/ZB.MOM.WW.MxGateway.Server/Sessions/SessionEventStreaming.cs

@@ -1,6 +1,7 @@
 using Microsoft.Extensions.Logging.Abstractions;
 using ZB.MOM.WW.MxGateway.Server.Configuration;
 using ZB.MOM.WW.MxGateway.Server.Grpc;
+using ZB.MOM.WW.MxGateway.Server.Metrics;
 
 namespace ZB.MOM.WW.MxGateway.Server.Sessions;
 
@@ -23,20 +24,28 @@ namespace ZB.MOM.WW.MxGateway.Server.Sessions;
 /// </param>
 /// <param name="DistributorLogger">Logger for the distributor pump lifecycle.</param>
 /// <param name="TimeProvider">Clock used to timestamp and age-evict replay entries.</param>
+/// <param name="Metrics">
+///     Gateway metrics sink used by the session's per-subscriber overflow handler to record
+///     the queue-overflow counter and, for legacy single-subscriber FailFast, the session
+///     fault. Carrying it here keeps the distributor decoupled from the metrics type while
+///     preserving the observability the pre-epic per-RPC overflow path emitted.
+/// </param>
 public sealed record SessionEventStreaming(
     MxAccessGrpcMapper Mapper,
     EventOptions EventOptions,
     ILogger<SessionEventDistributor> DistributorLogger,
-    TimeProvider TimeProvider)
+    TimeProvider TimeProvider,
+    GatewayMetrics Metrics)
 {
     /// <summary>
     ///     Defaults used when a session is constructed without explicit streaming
     ///     dependencies (unit tests). Uses a fresh mapper, default event options, a no-op
-    ///     logger, and the system clock.
+    ///     logger, the system clock, and a fresh metrics sink.
     /// </summary>
     public static SessionEventStreaming Default { get; } = new(
         new MxAccessGrpcMapper(),
         new EventOptions(),
         NullLogger<SessionEventDistributor>.Instance,
-        TimeProvider.System);
+        TimeProvider.System,
+        new GatewayMetrics());
 }