Add command and event payloads to activity

By exposing these as custom properties, we allow listeners to perform additional operations on them without having to extend the message bus itself.

src/Merq.Core/MessageBus.cs

@@ -190,7 +190,7 @@ public bool CanHandle(IExecutable command) => canHandleMap.GetOrAdd(GetCommandTy
     public void Execute(ICommand command)
     {
         var type = GetCommandType(command);
-        using var activity = StartActivity(type, Telemetry.Process);
+        using var activity = StartCommandActivity(type, command);
 
         try
         {
@@ -222,7 +222,7 @@ public void Execute(ICommand command)
     public TResult Execute<TResult>(ICommand<TResult> command)
     {
         var type = GetCommandType(command);
-        using var activity = StartActivity(type, Telemetry.Process);
+        using var activity = StartCommandActivity(type, command);
 
         try
         {
@@ -253,7 +253,7 @@ public TResult Execute<TResult>(ICommand<TResult> command)
     public Task ExecuteAsync(IAsyncCommand command, CancellationToken cancellation = default)
     {
         var type = GetCommandType(command);
-        using var activity = StartActivity(type, Telemetry.Process);
+        using var activity = StartCommandActivity(type, command);
 
         try
         {
@@ -286,7 +286,7 @@ public Task ExecuteAsync(IAsyncCommand command, CancellationToken cancellation =
     public Task<TResult> ExecuteAsync<TResult>(IAsyncCommand<TResult> command, CancellationToken cancellation = default)
     {
         var type = GetCommandType(command);
-        using var activity = StartActivity(type, Telemetry.Process);
+        using var activity = StartCommandActivity(type, command);
 
         try
         {
@@ -315,7 +315,7 @@ public Task<TResult> ExecuteAsync<TResult>(IAsyncCommand<TResult> command, Cance
     public void Notify<TEvent>(TEvent e)
     {
         var type = (e ?? throw new ArgumentNullException(nameof(e))).GetType();
-        using var activity = StartActivity(type, Publish);
+        using var activity = StartEventActivity(type, e);
         var watch = Stopwatch.StartNew();
 
         try

src/Merq.Core/Telemetry.cs

@@ -44,7 +44,12 @@ static Telemetry()
     //          NOTE: this is not an entirely satisfactory way to tell events from commands apart.
     public const string Process = nameof(Process);
 
-    public static Activity? StartActivity(Type type, string operation, [CallerMemberName] string? member = default, [CallerFilePath] string? file = default, [CallerLineNumber] int? line = default)
+    public static Activity? StartCommandActivity(Type type, object command) => StartActivity(type, Process, "Command", command);
+
+    public static Activity? StartEventActivity(Type type, object @event) => StartActivity(type, Publish, "Event", @event);
+
+    public static Activity? StartActivity(Type type, string operation, string? property = default, object? value = default,
+        [CallerMemberName] string? member = default, [CallerFilePath] string? file = default, [CallerLineNumber] int? line = default)
     {
         if (operation == Publish)
             events.Add(1, new KeyValuePair<string, object?>("Name", type.FullName));
@@ -55,7 +60,7 @@ static Telemetry()
         // Requirement is that the destination has low cardinality. In our case, the destination is 
         // the logical operation being performed, such as "Execute", "Notify" or "Deliver". The 
         // operation is actually the type being acted on (such as CreateUser -a command- or UserCreated -event).
-        return tracer.StartActivity(ActivityKind.Producer, name: $"{operation}/{type.FullName}")
+        var activity = tracer.CreateActivity($"{operation}/{type.FullName}", ActivityKind.Producer)
             ?.SetTag("code.function", member)
             ?.SetTag("code.filepath", file)
             ?.SetTag("code.lineno", line)
@@ -65,6 +70,13 @@ static Telemetry()
             ?.SetTag("messaging.operation", operation.ToLowerInvariant())
             ?.SetTag("messaging.protocol.name", type.Assembly.GetName().Name)
             ?.SetTag("messaging.protocol.version", type.Assembly.GetName().Version?.ToString() ?? "unknown");
+
+        if (property != null && value != null)
+            activity?.SetCustomProperty(property, value);
+
+        activity?.Start();
+
+        return activity;
     }
 
     public static void RecordException(this Activity? activity, Exception e)

src/Merq.Tests/MessageBusSpec.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.Reactive.Subjects;
 using System.Threading;
 using System.Threading.Tasks;
@@ -443,5 +444,61 @@ public async Task when_executing_non_public_àsynccommand_result_then_invokes_ha
         Assert.Equal(42, await bus.ExecuteAsync(new NonPublicAsyncCommandResult(), CancellationToken.None));
     }
 
+    [Fact]
+    public void when_notifying_can_access_event_from_activity_stop()
+    {
+        object? e = default;
+        using var listener = new ActivityListener
+        {
+            ActivityStopped = activity => e = activity.GetCustomProperty("Event"),
+            Sample = (ref ActivityCreationOptions<ActivityContext> _) => ActivitySamplingResult.AllData,
+            ShouldListenTo = source => source.Name == "Merq",
+        };
+
+        ActivitySource.AddActivityListener(listener);
+
+        bus.Notify(new ConcreteEvent());
+
+        Assert.NotNull(e);
+        Assert.IsType<ConcreteEvent>(e);
+    }
+
+    [Fact]
+    public void when_executing_can_access_event_from_activity_stop()
+    {
+        object? c = default;
+        using var listener = new ActivityListener
+        {
+            ActivityStopped = activity => c = activity.GetCustomProperty("Command"),
+            Sample = (ref ActivityCreationOptions<ActivityContext> _) => ActivitySamplingResult.AllData,
+            ShouldListenTo = source => source.Name == "Merq",
+        };
+
+        ActivitySource.AddActivityListener(listener);
+
+        Assert.Throws<InvalidOperationException>(() => bus.Execute(new Command()));
+
+        Assert.NotNull(c);
+    }
+
+    [Fact]
+    public void when_execute_throws_activity_has_error_status()
+    {
+        Activity? activity = default;
+        using var listener = new ActivityListener
+        {
+            ActivityStarted = x => activity = x,
+            Sample = (ref ActivityCreationOptions<ActivityContext> _) => ActivitySamplingResult.AllData,
+            ShouldListenTo = source => source.Name == "Merq",
+        };
+
+        ActivitySource.AddActivityListener(listener);
+
+        Assert.Throws<InvalidOperationException>(() => bus.Execute(new Command()));
+
+        Assert.NotNull(activity);
+        Assert.Equal(ActivityStatusCode.Error, activity.Status);
+    }
+
     class NestedEvent { }
 }

src/Samples/ConsoleApp/Program.cs

@@ -36,12 +36,14 @@
 var bus = services.GetRequiredService<IMessageBus>();
 
 // .NET-style activity listening
-//using var listener = new ActivityListener
-//{
-//    ActivityStarted = activity => MarkupLine($"[red]Activity started: {activity.OperationName}[/]"),
-//    ActivityStopped = activity => MarkupLine($"[red]Activity stopped: {activity.OperationName}[/]"),
-//    ShouldListenTo = source => source.Name == "Merq.Core",
-//};
+using var listener = new ActivityListener
+{
+    ActivityStarted = activity => MarkupLine($"[grey]Activity started: {activity.OperationName}[/]"),
+    ActivityStopped = activity => MarkupLine($"[grey]Activity stopped: {activity.OperationName}[/]"),
+    ShouldListenTo = source => source.Name == "Merq",
+};
+
+ActivitySource.AddActivityListener(listener);
 
 // Setup OpenTelemetry: https://learn.microsoft.com/en-us/dotnet/core/diagnostics/distributed-tracing-instrumentation-walkthroughs
 using var tracer = Sdk