feat: Add custom dimensions for metrics

src/OpenFeature.Contrib.Hooks.Otel/MetricHookCustomDimensions.cs

@@ -0,0 +1,29 @@
+using System.Collections.Generic;
+
+namespace OpenFeature.Contrib.Hooks.Otel
+{
+    /// <summary>
+    /// Represents a custom dimension list for a metric hook.
+    /// </summary>
+    public class MetricHookCustomDimensions
+    {
+        private readonly List<KeyValuePair<string, object>> _keyValuePairs = new List<KeyValuePair<string, object>>();
+
+        /// <summary>
+        /// Adds a custom dimension to the list.
+        /// </summary>
+        /// <param name="key">The key of the custom dimension.</param>
+        /// <param name="value">The value of the custom dimension.</param>
+        /// <returns>The custom dimension list.</returns>
+        public MetricHookCustomDimensions Add(string key, object value)
+        {
+            _keyValuePairs.Add(new KeyValuePair<string, object>(key, value));
+            return this;
+        }
+
+        internal KeyValuePair<string, object>[] GetTagList()
+        {
+            return _keyValuePairs.ToArray();
+        }
+    }
+}

src/OpenFeature.Contrib.Hooks.Otel/MetricsHook.cs

@@ -22,12 +22,15 @@ public class MetricsHook : Hook
         private readonly Counter<long> _evaluationRequestCounter;
         private readonly Counter<long> _evaluationSuccessCounter;
         private readonly Counter<long> _evaluationErrorCounter;
+        private readonly Func<FlagEvaluationDetails<T>, KeyValuePair<string, object>[]> _customDimensions;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="MetricsHook"/> class.
         /// </summary>
-        public MetricsHook()
+        /// <param name="customDimensions">The optional custom dimensions.</param>
+        public MetricsHook(Func<FlagEvaluationDetails<T>, KeyValuePair<string, object>[]> customDimensions = null)
         {
+            _customDimensions = customDimensions;
             var meter = new Meter(InstrumentationName, InstrumentationVersion);
 
             _evaluationActiveUpDownCounter = meter.CreateUpDownCounter<long>(MetricsConstants.ActiveCountName, description: MetricsConstants.ActiveDescription);
@@ -73,7 +76,8 @@ public override Task<EvaluationContext> Before<T>(HookContext<T> context, IReadO
         /// <returns>The evaluation context.</returns>
         public override Task After<T>(HookContext<T> context, FlagEvaluationDetails<T> details, IReadOnlyDictionary<string, object> hints = null)
         {
-            var tagList = new TagList
+            var initalTagList = _customDimensions != null ? _customDimensions(details) : Array.Empty<KeyValuePair<string, object>>();
+            var tagList = new TagList(initalTagList)
             {
                 { MetricsConstants.KeyAttr, context.FlagKey },
                 { MetricsConstants.ProviderNameAttr, context.ProviderMetadata.Name },

src/OpenFeature.Contrib.Hooks.Otel/OpenFeature.Contrib.Hooks.Otel.csproj

@@ -17,4 +17,8 @@
     <PackageReference Include="OpenTelemetry.Exporter.Console" Version="1.4.0" />
   </ItemGroup>
 
+  <ItemGroup>
+    <InternalsVisibleTo Include="OpenFeature.Contrib.Hooks.Otel.Test" />
+  </ItemGroup>
+
 </Project>

src/OpenFeature.Contrib.Hooks.Otel/README.md

@@ -72,12 +72,12 @@ For this hook to function correctly a global `MeterProvider` must be set.
 
 Below are the metrics extracted by this hook and dimensions they carry:
 
-| Metric key                             | Description                     | Unit         | Dimensions                          |
-| -------------------------------------- | ------------------------------- | ------------ | ----------------------------------- |
-| feature_flag.evaluation_requests_total | Number of evaluation requests   | {request}    | key, provider name                  |
-| feature_flag.evaluation_success_total  | Flag evaluation successes       | {impression} | key, provider name, reason, variant |
-| feature_flag.evaluation_error_total    | Flag evaluation errors          | Counter      | key, provider name                  |
-| feature_flag.evaluation_active_count   | Active flag evaluations counter | Counter      | key                                 |
+| Metric key                             | Description                     | Unit         | Dimensions                                               |
+| -------------------------------------- | ------------------------------- | ------------ | -------------------------------------------------------- |
+| feature_flag.evaluation_requests_total | Number of evaluation requests   | {request}    | key, provider name                                       |
+| feature_flag.evaluation_success_total  | Flag evaluation successes       | {impression} | key, provider name, reason, variant, custom dimensions\* |
+| feature_flag.evaluation_error_total    | Flag evaluation errors          | Counter      | key, provider name                                       |
+| feature_flag.evaluation_active_count   | Active flag evaluations counter | Counter      | key                                                      |
 
 Consider the following code example for usage.
 
@@ -125,6 +125,54 @@ namespace OpenFeatureTestApp
 
 After running this example, you should be able to see some metrics being generated into the console.
 
+### Custom dimensions
+
+The metrics hook can be enriched with custom dimensions. This is only available for the `feature_flag.evaluation_success_total` metric.
+In order to use it, you need to create a instance of the `MetricsHook` class with a dictionary of custom dimensions.
+
+See example:
+
+```csharp
+using OpenFeature.Contrib.Providers.Flagd;
+using OpenFeature;
+using OpenFeature.Contrib.Hooks.Otel;
+using OpenTelemetry;
+using OpenTelemetry.Metrics;
+
+namespace OpenFeatureTestApp
+{
+    class Hello {
+        static void Main(string[] args) {
+
+            // set up the OpenTelemetry OTLP exporter
+            var meterProvider = Sdk.CreateMeterProviderBuilder()
+                    .AddMeter("OpenFeature.Contrib.Hooks.Otel")
+                    .ConfigureResource(r => r.AddService("openfeature-test"))
+                    .AddConsoleExporter()
+                    .Build();
+
+            // add the Otel Hook to the OpenFeature instance
+            var options = new MetricHookCustomDimensions()
+                .Add("key", "value")
+                .Add("test", "test");
+            OpenFeature.Api.Instance.AddHooks(new MetricsHook(options));
+
+            var flagdProvider = new FlagdProvider(new Uri("http://localhost:8013"));
+
+            // Set the flagdProvider as the provider for the OpenFeature SDK
+            OpenFeature.Api.Instance.SetProvider(flagdProvider);
+
+            var client = OpenFeature.Api.Instance.GetClient("my-app");
+
+            var val = client.GetBooleanValue("myBoolFlag", false, null);
+
+            // Print the value of the 'myBoolFlag' feature flag
+            System.Console.WriteLine(val.Result.ToString());
+        }
+    }
+}
+```
+
 ## License
 
 Apache 2.0 - See [LICENSE](./../../LICENSE) for more information.

test/OpenFeature.Contrib.Hooks.Otel.Test/MetricHookCustomDimensionsTest.cs

@@ -0,0 +1,38 @@
+using Xunit;
+
+namespace OpenFeature.Contrib.Hooks.Otel.Test
+{
+    public class MetricHookCustomDimensionsTest
+    {
+        [Fact]
+        public void Adds_CustomDimension_HasValues()
+        {
+            // Arrange
+            var customDimensions = new MetricHookCustomDimensions();
+            string key = "dimensionKey";
+            object value = "dimensionValue";
+
+            // Act
+            customDimensions.Add(key, value);
+
+            // Assert
+            var tagList = customDimensions.GetTagList();
+            Assert.Single(tagList);
+            Assert.Equal(key, tagList[0].Key);
+            Assert.Equal(value, tagList[0].Value);
+        }
+
+        [Fact]
+        public void CustomDimensionToList_IsEmpty()
+        {
+            // Arrange
+            var customDimensions = new MetricHookCustomDimensions();
+
+            // Act
+
+            // Assert
+            var tagList = customDimensions.GetTagList();
+            Assert.Empty(tagList);
+        }
+    }
+}

test/OpenFeature.Contrib.Hooks.Otel.Test/OpenFeature.Contrib.Hooks.Otel.Test.csproj

@@ -4,9 +4,8 @@
         <ProjectReference Include="..\..\src\OpenFeature.Contrib.Hooks.Otel\OpenFeature.Contrib.Hooks.Otel.csproj" />
     </ItemGroup>
 
-    <ItemGroup>
-      <PackageReference Include="OpenTelemetry" Version="1.4.0" />
-      <PackageReference Include="OpenTelemetry.Exporter.InMemory" Version="1.4.0" />
+    <ItemGroup>
+      <PackageReference Include="OpenTelemetry.Exporter.InMemory" Version="1.4.0" />
     </ItemGroup>
 
 </Project>