Introduce TelemetryOptions.OnTelemetryEvent (#1387)

src/Polly.Core/Telemetry/ResilienceTelemetrySource.cs

@@ -1,5 +1,3 @@
-using System.ComponentModel;
-
 namespace Polly.Telemetry;
 
 /// <summary>
@@ -12,7 +10,6 @@ namespace Polly.Telemetry;
 /// <remarks>
 /// This class is used by the telemetry infrastructure and should not be used directly by user code.
 /// </remarks>
-[EditorBrowsable(EditorBrowsableState.Never)]
 public sealed record class ResilienceTelemetrySource(
     string? BuilderName,
     ResilienceProperties BuilderProperties,

src/Polly.Core/Telemetry/TelemetryEventArguments.cs

@@ -1,11 +1,8 @@
-using System.ComponentModel;
-
 namespace Polly.Telemetry;
 
 /// <summary>
 /// The arguments of the telemetry event.
 /// </summary>
-[EditorBrowsable(EditorBrowsableState.Never)]
 public sealed partial record class TelemetryEventArguments
 {
     private TelemetryEventArguments()

src/Polly.Extensions/Telemetry/ResilienceTelemetryDiagnosticSource.cs

@@ -10,13 +10,15 @@ internal class ResilienceTelemetryDiagnosticSource : DiagnosticSource
 
     private readonly ILogger _logger;
     private readonly Func<ResilienceContext, object?, object?> _resultFormatter;
+    private readonly Action<TelemetryEventArguments>? _onEvent;
     private readonly List<Action<EnrichmentContext>> _enrichers;
 
     public ResilienceTelemetryDiagnosticSource(TelemetryOptions options)
     {
         _enrichers = options.Enrichers.ToList();
         _logger = options.LoggerFactory.CreateLogger(TelemetryUtil.PollyDiagnosticSource);
         _resultFormatter = options.ResultFormatter;
+        _onEvent = options.OnTelemetryEvent;
 
         Counter = Meter.CreateCounter<int>(
             "resilience-events",
@@ -41,6 +43,8 @@ public override void Write(string name, object? value)
             return;
         }
 
+        _onEvent?.Invoke(args);
+
         LogEvent(args);
         MeterEvent(args);
     }

src/Polly.Extensions/Telemetry/TelemetryOptions.cs

@@ -2,6 +2,7 @@
 using System.Net.Http;
 using Microsoft.Extensions.Logging;
 using Microsoft.Extensions.Logging.Abstractions;
+using Polly.Telemetry;
 
 namespace Polly.Extensions.Telemetry;
 
@@ -10,6 +11,14 @@ namespace Polly.Extensions.Telemetry;
 /// </summary>
 public class TelemetryOptions
 {
+    /// <summary>
+    /// Gets or sets the callback that is raised when <see cref="TelemetryEventArguments"/> is received from Polly.
+    /// </summary>
+    /// <remarks>
+    /// Defaults to <see langword="null"/>.
+    /// </remarks>
+    public Action<TelemetryEventArguments>? OnTelemetryEvent { get; set; }
+
     /// <summary>
     /// Gets or sets the logger factory.
     /// </summary>

test/Polly.Extensions.Tests/Telemetry/ResilienceTelemetryDiagnosticSourceTests.cs

@@ -14,6 +14,7 @@ public class ResilienceTelemetryDiagnosticSourceTests : IDisposable
     private readonly ILoggerFactory _loggerFactory;
     private readonly List<MeteringEvent> _events = new();
     private readonly IDisposable _metering;
+    private Action<TelemetryEventArguments>? _onEvent;
 
     public ResilienceTelemetryDiagnosticSourceTests()
     {
@@ -319,13 +320,32 @@ public void WriteEvent_MeteringWithoutStrategyKey_Ok()
         var events = GetEvents("resilience-events")[0]["strategy-key"].Should().BeNull();
     }
 
+    [InlineData(true)]
+    [InlineData(false)]
+    [Theory]
+    public void OnTelemetryEvent_Ok(bool hasCallback)
+    {
+        var called = false;
+
+        if (hasCallback)
+        {
+            _onEvent = e => called = true;
+        }
+
+        var telemetry = Create();
+        ReportEvent(telemetry, null, strategyKey: null);
+
+        called.Should().Be(hasCallback);
+    }
+
     private List<Dictionary<string, object?>> GetEvents(string eventName) => _events.Where(e => e.Name == eventName).Select(v => v.Tags).ToList();
 
     private ResilienceTelemetryDiagnosticSource Create(Action<ICollection<Action<EnrichmentContext>>>? configureEnrichers = null)
     {
         var options = new TelemetryOptions
         {
-            LoggerFactory = _loggerFactory
+            LoggerFactory = _loggerFactory,
+            OnTelemetryEvent = _onEvent
         };
 
         configureEnrichers?.Invoke(options.Enrichers);