fix(grpc): count a worker fault once, not once per StreamEvents subscriber

WorkerClient.SetFaulted already records mxgateway.faults; EventStreamService added a
WorkerFaulted fault per subscriber, so one fault read as 1+K. The subscriber now records
only its WorkerFault disconnect reason. docs/Metrics.md updated.

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

@@ -174,12 +174,13 @@ public sealed class EventStreamService(
                 {
                     // The distributor pump completes every subscriber channel with the source
                     // fault when the worker event stream terminates abnormally; that surfaces
-                    // here. Mirror the original ProduceEventsAsync behavior: fault the
-                    // session and record the metric, then propagate the terminal fault to the
-                    // gRPC client.
+                    // here. Fault the session and propagate the terminal fault to the gRPC
+                    // client. The fault itself is NOT counted here: WorkerClient.SetFaulted
+                    // already recorded it once, and every subscriber of the session sees the
+                    // same fault, so counting it per subscriber inflated mxgateway.faults to
+                    // 1+K. This subscriber's share is the WorkerFault disconnect reason.
                     disconnectReason = EventStreamDisconnectReasons.WorkerFault;
                     session.MarkFaulted(workerException.Message);
-                    metrics.Fault(WorkerClientErrorCode.WorkerFaulted.ToString());
                     throw;
                 }
                 catch (SessionManagerException sessionException)