perf(metrics): cache MxEventFamily names for per-event metric tags

WorkerClient.StageWorkerEvent and StreamEvents called Enum.ToString per
event (per subscriber on the stream path). MxEventFamilyNames serves the
contract-fixed names from a frozen lookup, falling back to ToString for
undefined values so exported tag values are unchanged.

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

@@ -183,7 +183,7 @@ public sealed class MxAccessGatewayService(
                 // call is cancelled, instead of leaving it to the transport's own timeout.
                 await responseStream.WriteAsync(publicEvent, context.CancellationToken).ConfigureAwait(false);
                 metrics.RecordEventStreamSend(
-                    publicEvent.Family.ToString(),
+                    MxEventFamilyNames.Get(publicEvent.Family),
                     Stopwatch.GetElapsedTime(sendStartTimestamp));
             }
         }

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

@@ -0,0 +1,28 @@
+using System.Collections.Frozen;
+using ZB.MOM.WW.MxGateway.Contracts.Proto;
+
+namespace ZB.MOM.WW.MxGateway.Server.Metrics;
+
+/// <summary>
+///     Cached <see cref="MxEventFamily"/> names for the per-event metric tags.
+/// </summary>
+/// <remarks>
+///     <see cref="Enum.ToString()"/> allocates a new string on every call, and the event-received
+///     and stream-send metrics tag every event with its family — once per event at the worker
+///     boundary and once per event per subscriber on the stream. The names are fixed by the
+///     contract, so they are computed once here. A value the contract does not define falls back
+///     to <see cref="Enum.ToString()"/>, which is the same string the uncached call produced.
+/// </remarks>
+public static class MxEventFamilyNames
+{
+    private static readonly FrozenDictionary<MxEventFamily, string> Names =
+        Enum.GetValues<MxEventFamily>()
+            .Distinct()
+            .ToFrozenDictionary(family => family, family => family.ToString());
+
+    /// <summary>Gets the name of an event family.</summary>
+    /// <param name="family">The event family.</param>
+    /// <returns>The family's name, identical to <see cref="Enum.ToString()"/>.</returns>
+    public static string Get(MxEventFamily family) =>
+        Names.TryGetValue(family, out string? name) ? name : family.ToString();
+}

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

@@ -692,7 +692,7 @@ public sealed class WorkerClient : IWorkerClient
     {
         if (workerEvent.Event is not null)
         {
-            _metrics?.EventReceived(SessionId, workerEvent.Event.Family.ToString());
+            _metrics?.EventReceived(SessionId, MxEventFamilyNames.Get(workerEvent.Event.Family));
         }
 
         if (_eventStaging.Writer.TryWrite(workerEvent))

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

@@ -0,0 +1,41 @@
+using ZB.MOM.WW.MxGateway.Contracts.Proto;
+using ZB.MOM.WW.MxGateway.Server.Metrics;
+
+namespace ZB.MOM.WW.MxGateway.Tests.Metrics;
+
+public sealed class MxEventFamilyNamesTests
+{
+    /// <summary>
+    ///     Every defined family maps to exactly the name <see cref="Enum.ToString()"/> produces, so
+    ///     switching the per-event metric tags to the cached lookup changes no exported tag value.
+    /// </summary>
+    [Fact]
+    public void Get_ForEveryDefinedFamily_MatchesEnumToString()
+    {
+        foreach (MxEventFamily family in Enum.GetValues<MxEventFamily>())
+        {
+            Assert.Equal(family.ToString(), MxEventFamilyNames.Get(family));
+        }
+    }
+
+    /// <summary>A defined family returns the same cached string instance on every call (no per-event allocation).</summary>
+    [Fact]
+    public void Get_ForDefinedFamily_ReturnsCachedInstance()
+    {
+        Assert.Same(
+            MxEventFamilyNames.Get(MxEventFamily.OnDataChange),
+            MxEventFamilyNames.Get(MxEventFamily.OnDataChange));
+    }
+
+    /// <summary>
+    ///     A numeric value the contract does not define (a newer worker, or a malformed frame) still
+    ///     produces the <see cref="Enum.ToString()"/> form rather than throwing.
+    /// </summary>
+    [Fact]
+    public void Get_ForUndefinedValue_FallsBackToToString()
+    {
+        MxEventFamily undefined = (MxEventFamily)987654;
+
+        Assert.Equal(undefined.ToString(), MxEventFamilyNames.Get(undefined));
+    }
+}