update to sync GetService method

Author: zhiyuanliang-ms

src/Microsoft.FeatureManagement/IVariantServiceProvider.cs

@@ -14,8 +14,7 @@ public interface IVariantServiceProvider<TService> where TService : class
         /// <summary>
         /// Gets an implementation variant of TService.
         /// </summary>
-        /// <param name="cancellationToken">The cancellation token to cancel the operation.</param>
         /// <returns>An implementation of TService.</returns>
-        ValueTask<TService> GetAsync(CancellationToken cancellationToken);
+        TService GetService();
     }
 }

src/Microsoft.FeatureManagement/VariantServiceProvider.cs

@@ -42,13 +42,12 @@ public string VariantFeatureName
         /// <summary>
         /// Gets implementation of TService according to the assigned variant from the feature flag.
         /// </summary>
-        /// <param name="cancellationToken">The cancellation token to cancel the operation.</param>
         /// <returns>An implementation matched with the assigned variant. If there is no matched implementation, it will return null.</returns>
-        public async ValueTask<TService> GetAsync(CancellationToken cancellationToken)
+        public TService GetService()
         {
             Debug.Assert(_variantFeatureName != null);
 
-            Variant variant = await _featureManager.GetVariantAsync(_variantFeatureName, cancellationToken);
+            Variant variant = _featureManager.GetVariantAsync(_variantFeatureName, CancellationToken.None).GetAwaiter().GetResult(); ;
 
             TService implementation = null;