Merges variants and isenabled paths. Adds variant reason field. (#290)

* Adds Reason field and adjusts evaluation logic

* use enum AssignmentReason

* add more testcases & remove some interal methods

* fix typo

* update comments

* resolve comments

* update comment for DisabledDefault

* fix typo

* update

* update telemetry publisher

* add reason when no allocation section

* update

* remove nullable

* resolve comments

* update comments

---------

Co-authored-by: zhiyuanliang <zhiyuanliang@microsoft.com>

Author: rossgrambo

src/Microsoft.FeatureManagement.Telemetry.ApplicationInsights/ApplicationInsightsTelemetryPublisher.cs

@@ -2,6 +2,7 @@
 // Licensed under the MIT license.
 //
 using Microsoft.ApplicationInsights;
+using System.Diagnostics;
 
 namespace Microsoft.FeatureManagement.Telemetry.ApplicationInsights
 {
@@ -30,15 +31,17 @@ public ValueTask PublishEvent(EvaluationEvent evaluationEvent, CancellationToken
 
             FeatureDefinition featureDefinition = evaluationEvent.FeatureDefinition;
 
-            Dictionary<string, string> properties = new Dictionary<string, string>()
+            var properties = new Dictionary<string, string>()
             {
                 { "FeatureName", featureDefinition.Name },
-                { "IsEnabled", evaluationEvent.IsEnabled.ToString() }
+                { "Enabled", evaluationEvent.Enabled.ToString() }
             };
 
-            if (evaluationEvent.Variant != null)
+            if (evaluationEvent.VariantAssignmentReason != VariantAssignmentReason.None)
             {
-                properties["Variant"] = evaluationEvent.Variant.Name;
+                properties["Variant"] = evaluationEvent.Variant?.Name;
+
+                properties["VariantAssignmentReason"] = ToString(evaluationEvent.VariantAssignmentReason);
             }
 
             if (featureDefinition.TelemetryMetadata != null)
@@ -66,5 +69,26 @@ private void ValidateEvent(EvaluationEvent evaluationEvent)
                 throw new ArgumentNullException(nameof(evaluationEvent.FeatureDefinition));
             }
         }
+
+        private static string ToString(VariantAssignmentReason reason)
+        {
+            Debug.Assert(reason != VariantAssignmentReason.None);
+
+            const string DefaultWhenDisabled = "DefaultWhenDisabled";
+            const string DefaultWhenEnabled = "DefaultWhenEnabled";
+            const string User = "User";
+            const string Group = "Group";
+            const string Percentile = "Percentile";
+
+            return reason switch
+            {
+                VariantAssignmentReason.DefaultWhenDisabled => DefaultWhenDisabled,
+                VariantAssignmentReason.DefaultWhenEnabled => DefaultWhenEnabled,
+                VariantAssignmentReason.User => User,
+                VariantAssignmentReason.Group => Group,
+                VariantAssignmentReason.Percentile => Percentile,
+                _ => throw new ArgumentException("Invalid assignment reason.", nameof(reason))
+            };
+        }
     }
 }

src/Microsoft.FeatureManagement.Telemetry.ApplicationInsights/Microsoft.FeatureManagement.Telemetry.ApplicationInsights.csproj

@@ -3,7 +3,6 @@
   <PropertyGroup>
     <TargetFramework>net6.0</TargetFramework>
     <ImplicitUsings>enable</ImplicitUsings>
-    <Nullable>enable</Nullable>
   </PropertyGroup>
 
   <ItemGroup>

src/Microsoft.FeatureManagement/FeatureManager.cs

@@ -1,8 +1,6 @@
 // Copyright (c) Microsoft Corporation.
 // Licensed under the MIT license.
 //
-using Microsoft.FeatureManagement.FeatureFilters;
-
 namespace Microsoft.FeatureManagement.Telemetry
 {
     /// <summary>
@@ -18,11 +16,16 @@ public class EvaluationEvent
         /// <summary>
         /// The enabled state of the feature after evaluation.
         /// </summary>
-        public bool IsEnabled { get; set; }
+        public bool Enabled { get; set; }
 
         /// <summary>
         /// The variant given after evaluation.
         /// </summary>
         public Variant Variant { get; set; }
+
+        /// <summary>
+        /// The reason why the variant was assigned.
+        /// </summary>
+        public VariantAssignmentReason VariantAssignmentReason { get; set; }
     }
 }

src/Microsoft.FeatureManagement/Telemetry/EvaluationEvent.cs

@@ -0,0 +1,41 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT license.
+//
+namespace Microsoft.FeatureManagement.Telemetry
+{
+    /// <summary>
+    /// The reason the variant was assigned during the evaluation of a feature.
+    /// </summary>
+    public enum VariantAssignmentReason
+    {
+        /// <summary>
+        /// Variant allocation did not happend. No variant is assigned.
+        /// </summary>
+        None,
+
+        /// <summary>
+        /// The default variant is assigned when a feature flag is disabled.
+        /// </summary>
+        DefaultWhenDisabled,
+
+        /// <summary>
+        /// The default variant is assigned because of no applicable user/group/percentile allocation when a feature flag is enabled.
+        /// </summary>
+        DefaultWhenEnabled,
+
+        /// <summary>
+        /// The variant is assigned because of the user allocation when a feature flag is enabled.
+        /// </summary>
+        User,
+
+        /// <summary>
+        /// The variant is assigned because of the group allocation when a feature flag is enabled.
+        /// </summary>
+        Group,
+
+        /// <summary>
+        /// The variant is assigned because of the percentile allocation when a feature flag is enabled.
+        /// </summary>
+        Percentile
+    }
+}