Merge feat/telemetry-followons: telemetry follow-ons for MxAccessGateway

Metric normalization: meter MxGateway.Server -> ZB.MOM.WW.MxGateway and the 3
duration histograms ms -> s (safe: never Prometheus-exported before). Config-driven
OTLP exporter opt-in (default Prometheus). Metrics.md synced; doc-review artifacts
gitignored.

.gitignore

@@ -147,3 +147,8 @@ generated-scratch/
 
 # Keep empty directories with .gitkeep files when needed
 !.gitkeep
+
+# Documentation review artifacts (CommentChecker output)
+*-docs-issues.md
+*-docs-fixed.md
+*-docs-final.md

docs/Metrics.md

@@ -4,7 +4,7 @@ The metrics subsystem exposes counters, histograms, and observable gauges that d
 
 ## Overview
 
-`GatewayMetrics` is a singleton (registered in `GatewayApplication.cs`) that owns a single `Meter` named `ZB.MOM.WW.MxGateway.Server` and a set of synchronised counters, histograms, and observable gauges. Subsystems call typed mutator methods (`SessionOpened`, `CommandFailed`, `EventReceived`, etc.) rather than touching the `Meter` directly, which keeps the OpenTelemetry instrument names and tag conventions in one place. A `lock (_syncRoot)` block guards the scalar fields used by `GetSnapshot`, while per-event maps use `ConcurrentDictionary<string, long>` so the hot event path avoids the lock.
+`GatewayMetrics` is a singleton (registered in `GatewayApplication.cs`) that owns a single `Meter` named `ZB.MOM.WW.MxGateway` and a set of synchronised counters, histograms, and observable gauges. Subsystems call typed mutator methods (`SessionOpened`, `CommandFailed`, `EventReceived`, etc.) rather than touching the `Meter` directly, which keeps the OpenTelemetry instrument names and tag conventions in one place. A `lock (_syncRoot)` block guards the scalar fields used by `GetSnapshot`, while per-event maps use `ConcurrentDictionary<string, long>` so the hot event path avoids the lock.
 
 ## Meter and OpenTelemetry Compatibility
 
@@ -13,7 +13,7 @@ The meter name is exposed as a constant so that hosting code can register it wit
 ```csharp
 public sealed class GatewayMetrics : IDisposable
 {
-    public const string MeterName = "ZB.MOM.WW.MxGateway.Server";
+    public const string MeterName = "ZB.MOM.WW.MxGateway";
 
     public GatewayMetrics()
     {
@@ -50,12 +50,12 @@ All counters are `Counter<long>`. Tag values come from the call sites listed und
 
 ### Histograms
 
-Histograms record durations in milliseconds (the `unit` argument on `CreateHistogram`):
+Histograms record durations in seconds (the `unit` argument on `CreateHistogram`):
 
 ```csharp
-_workerStartupLatencyHistogram = _meter.CreateHistogram<double>("mxgateway.workers.startup.duration", "ms");
-_commandLatencyHistogram = _meter.CreateHistogram<double>("mxgateway.commands.duration", "ms");
-_eventStreamSendLatencyHistogram = _meter.CreateHistogram<double>("mxgateway.events.stream_send.duration", "ms");
+_workerStartupLatencyHistogram = _meter.CreateHistogram<double>("mxgateway.workers.startup.duration", "s");
+_commandLatencyHistogram = _meter.CreateHistogram<double>("mxgateway.commands.duration", "s");
+_eventStreamSendLatencyHistogram = _meter.CreateHistogram<double>("mxgateway.events.stream_send.duration", "s");
 ```
 
 | Instrument | Tags | What it measures |

src/ZB.MOM.WW.MxGateway.Server/GatewayApplication.cs

@@ -78,6 +78,11 @@ public static class GatewayApplication
         {
             o.ServiceName = "mxgateway";
             o.Meters = [GatewayMetrics.MeterName]; // "MxGateway.Server" — name unchanged
+            if (Enum.TryParse<ZbExporter>(builder.Configuration["MxGateway:Telemetry:Exporter"], ignoreCase: true, out var exporter))
+                o.Exporter = exporter;
+            var otlp = builder.Configuration["MxGateway:Telemetry:OtlpEndpoint"];
+            if (!string.IsNullOrWhiteSpace(otlp))
+                o.OtlpEndpoint = otlp;
         });
         builder.Services.AddSingleton<ILogRedactor, GatewayLogRedactorSeam>();
         builder.Services.AddSingleton<MxAccessGrpcMapper>();

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

@@ -5,7 +5,7 @@ namespace ZB.MOM.WW.MxGateway.Server.Metrics;
 
 public sealed class GatewayMetrics : IDisposable
 {
-    public const string MeterName = "MxGateway.Server";
+    public const string MeterName = "ZB.MOM.WW.MxGateway";
 
     private readonly object _syncRoot = new();
     private readonly Meter _meter;
@@ -68,9 +68,9 @@ public sealed class GatewayMetrics : IDisposable
         _heartbeatFailuresCounter = _meter.CreateCounter<long>("mxgateway.heartbeats.failed");
         _streamDisconnectsCounter = _meter.CreateCounter<long>("mxgateway.grpc.streams.disconnected");
         _retryAttemptsCounter = _meter.CreateCounter<long>("mxgateway.retries.attempted");
-        _workerStartupLatencyHistogram = _meter.CreateHistogram<double>("mxgateway.workers.startup.duration", "ms");
-        _commandLatencyHistogram = _meter.CreateHistogram<double>("mxgateway.commands.duration", "ms");
-        _eventStreamSendLatencyHistogram = _meter.CreateHistogram<double>("mxgateway.events.stream_send.duration", "ms");
+        _workerStartupLatencyHistogram = _meter.CreateHistogram<double>("mxgateway.workers.startup.duration", "s");
+        _commandLatencyHistogram = _meter.CreateHistogram<double>("mxgateway.commands.duration", "s");
+        _eventStreamSendLatencyHistogram = _meter.CreateHistogram<double>("mxgateway.events.stream_send.duration", "s");
 
         _meter.CreateObservableGauge("mxgateway.sessions.open", GetOpenSessions);
         _meter.CreateObservableGauge("mxgateway.workers.running", GetWorkersRunning);
@@ -144,7 +144,7 @@ public sealed class GatewayMetrics : IDisposable
             _workersRunning++;
         }
 
-        _workerStartupLatencyHistogram.Record(startupDuration.TotalMilliseconds);
+        _workerStartupLatencyHistogram.Record(startupDuration.TotalSeconds);
     }
 
     /// <summary>
@@ -208,7 +208,7 @@ public sealed class GatewayMetrics : IDisposable
 
         KeyValuePair<string, object?> methodTag = new("method", method);
         _commandsSucceededCounter.Add(1, methodTag);
-        _commandLatencyHistogram.Record(duration.TotalMilliseconds, methodTag);
+        _commandLatencyHistogram.Record(duration.TotalSeconds, methodTag);
     }
 
     /// <summary>
@@ -228,7 +228,7 @@ public sealed class GatewayMetrics : IDisposable
         KeyValuePair<string, object?> methodTag = new("method", method);
         KeyValuePair<string, object?> categoryTag = new("category", category);
         _commandsFailedCounter.Add(1, methodTag, categoryTag);
-        _commandLatencyHistogram.Record(duration.TotalMilliseconds, methodTag, categoryTag);
+        _commandLatencyHistogram.Record(duration.TotalSeconds, methodTag, categoryTag);
     }
 
     /// <summary>
@@ -255,7 +255,7 @@ public sealed class GatewayMetrics : IDisposable
     public void RecordEventStreamSend(string family, TimeSpan duration)
     {
         _eventStreamSendLatencyHistogram.Record(
-            duration.TotalMilliseconds,
+            duration.TotalSeconds,
             new KeyValuePair<string, object?>("family", family));
     }