Add telemetry for NUGET_USE_LEGACY_ASSET_TARGET_FALLBACK_DEPENDENCY_RESOLUTION (#7231)

Author: nkolev92

src/NuGet.Core/NuGet.Commands/RestoreCommand/RestoreCommand.cs

@@ -40,6 +40,7 @@ public class RestoreCommand
         private readonly Dictionary<RestoreTargetGraph, Dictionary<string, LibraryIncludeFlags>> _includeFlagGraphs
             = new Dictionary<RestoreTargetGraph, Dictionary<string, LibraryIncludeFlags>>();
 
+        internal IEnvironmentVariableReader EnvironmentVariableReader { get; init; }
         public Guid ParentId { get; }
 
         private const string ProjectRestoreInformation = nameof(ProjectRestoreInformation);
@@ -67,6 +68,7 @@ private readonly Dictionary<RestoreTargetGraph, Dictionary<string, LibraryInclud
         private const string UpdatedMSBuildFiles = nameof(UpdatedMSBuildFiles);
         private const string IsPackageInstallationTrigger = nameof(IsPackageInstallationTrigger);
         private const string UsesLegacyPackagesDirectory = nameof(UsesLegacyPackagesDirectory);
+        private const string UsesLegacyAssetTargetFallback = nameof(UsesLegacyAssetTargetFallback);
 
         // no-op data names
         private const string NoOpDuration = nameof(NoOpDuration);
@@ -173,6 +175,7 @@ public RestoreCommand(RestoreRequest request)
 
             _isLockFileEnabled = PackagesLockFileUtilities.IsNuGetLockFileEnabled(_request.Project);
             _enableNewDependencyResolver = _request.Project.RuntimeGraph.Supports.Count == 0 && ShouldUseNewResolverWithLockFile(_isLockFileEnabled, _request.Project) && !_request.Project.RestoreMetadata.UseLegacyDependencyResolver;
+            EnvironmentVariableReader = EnvironmentVariableWrapper.Instance;
         }
 
         // Use the new resolver if lock files are not enabled, or if lock files are enabled and legacy projects or .NET 10 SDK is used.
@@ -384,6 +387,8 @@ private void InitializeTelemetry(TelemetryActivity telemetry, int httpSourcesCou
             telemetry.TelemetryEvent[NETSdkVersion] = _request.Project.RestoreSettings.SdkVersion;
             telemetry.TelemetryEvent[IsPackageInstallationTrigger] = !_request.IsRestoreOriginalAction;
             telemetry.TelemetryEvent[UsesLegacyPackagesDirectory] = !_request.IsLowercasePackagesDirectory;
+            telemetry.TelemetryEvent[UsesLegacyAssetTargetFallback] = MSBuildStringUtility.IsTrue(EnvironmentVariableReader.GetEnvironmentVariable("NUGET_USE_LEGACY_ASSET_TARGET_FALLBACK_DEPENDENCY_RESOLUTION"));
+
             _operationId = telemetry.OperationId;
 
             var isCpvmEnabled = _request.Project.RestoreMetadata?.CentralPackageVersionsEnabled ?? false;

test/NuGet.Core.Tests/NuGet.Commands.Test/RestoreCommandTests/RestoreCommandTests.cs

@@ -3022,6 +3022,7 @@ await SimpleTestPackageUtility.CreateFolderFeedV3Async(
                 ["Pruning.RemovablePackages.Count"] = value => value.Should().BeOfType<int>(),
                 ["Pruning.Pruned.Direct.Count"] = value => value.Should().BeOfType<int>(),
                 ["UsesLegacyPackagesDirectory"] = value => value.Should().Be(false),
+                ["UsesLegacyAssetTargetFallback"] = value => value.Should().Be(false),
             };
 
             HashSet<string> actualProperties = new();
@@ -3134,7 +3135,7 @@ await SimpleTestPackageUtility.CreateFolderFeedV3Async(
 
             var projectInformationEvent = telemetryEvents.Single(e => e.Name.Equals("ProjectRestoreInformation"));
 
-            projectInformationEvent.Count.Should().Be(40);
+            projectInformationEvent.Count.Should().Be(41);
 
             projectInformationEvent["RestoreSuccess"].Should().Be(true);
             projectInformationEvent["NoOpResult"].Should().Be(true);
@@ -3176,6 +3177,7 @@ await SimpleTestPackageUtility.CreateFolderFeedV3Async(
             projectInformationEvent["Pruning.FrameworksUnsupported.Count"].Should().Be(1);
             projectInformationEvent["Pruning.DefaultEnabled"].Should().Be(false);
             projectInformationEvent["UsesLegacyPackagesDirectory"].Should().Be(false);
+            projectInformationEvent["UsesLegacyAssetTargetFallback"].Should().Be(false);
         }
 
         [Fact]
@@ -3233,7 +3235,7 @@ await SimpleTestPackageUtility.CreateFolderFeedV3Async(
 
             var projectInformationEvent = telemetryEvents.Single(e => e.Name.Equals("ProjectRestoreInformation"));
 
-            projectInformationEvent.Count.Should().Be(48);
+            projectInformationEvent.Count.Should().Be(49);
             projectInformationEvent["RestoreSuccess"].Should().Be(true);
             projectInformationEvent["NoOpResult"].Should().Be(false);
             projectInformationEvent["TotalUniquePackagesCount"].Should().Be(2);