Add unit test for GetIndexMetrics

Author: leminh98

Microsoft.Azure.Cosmos/src/Linq/CosmosLinqExtensions.cs

@@ -591,9 +591,8 @@ public static FeedIterator ToStreamIterator<T>(this IQueryable<T> query)
         }
 
         /// <summary>
-        /// This extension method returns the Index Metrics for a given Response object. The index utilization metrics is to be used for debugging purposes. 
-        /// It's applicable to query response only.
-        /// This result is only available if QueryRequestOptions.PopulateIndexMetrics is set to true.
+        /// This extension method returns the Index Metrics for a given Response object. The index utilization metrics is to be used for debugging purposes only. 
+        /// This result is only available if QueryRequestOptions.PopulateIndexMetrics is set to true.Returns null if the index metrics is not available in the response.
         /// </summary>
         /// <param name="response">The query Response.</param>
         /// <returns>A string represents the Index Metrics.</returns>
@@ -609,7 +608,7 @@ public static FeedIterator ToStreamIterator<T>(this IQueryable<T> query)
         /// </example>
         public static string GetIndexMetrics<T>(this Response<T> response)
         {
-            return ResponseMessage.DecodeIndexMetrics(response.Headers, isBase64Encoded: false).Value;
+            return ResponseMessage.DecodeIndexMetrics(response.Headers, isBase64Encoded: false)?.Value;
         }
 
         /// <summary>

Microsoft.Azure.Cosmos/tests/Microsoft.Azure.Cosmos.EmulatorTests/Linq/CosmosItemLinqTests.cs

@@ -396,6 +396,34 @@ public async Task QueryableExtentionFunctionsTest()
             Assert.AreEqual(100, maxTaskNum);
         }
 
+
+        [TestMethod]
+        public async Task GetIndexMetricsTest()
+        {
+            //Creating items for query.
+            IList<ToDoActivity> itemList = await ToDoActivity.CreateRandomItems(container: this.Container, pkCount: 10, perPKItemCount: 1, randomPartitionKey: true);
+
+            QueryRequestOptions queryRequestOptions = new QueryRequestOptions() { PopulateIndexMetrics = true };
+            IOrderedQueryable<ToDoActivity> linqQueryable = this.Container.GetItemLinqQueryable<ToDoActivity>(
+                requestOptions: queryRequestOptions);
+
+            // Response object with valid index metrics field
+            Response<int> response = await linqQueryable.Select(item => item.taskNum).SumAsync();
+            this.VerifyResponse(response, 420, queryRequestOptions);
+
+            string indexMetrics = response.GetIndexMetrics();
+            Assert.AreEqual(
+                @"{""UtilizedIndexes"":{""SingleIndexes"":[{""IndexSpec"":""/taskNum/?""}],""CompositeIndexes"":[]},""PotentialIndexes"":{""SingleIndexes"":[],""CompositeIndexes"":[]}}",
+                indexMetrics);
+
+            // Response object with null index metrics field]
+            response.Headers.IndexUtilizationText = null;
+            indexMetrics = response.GetIndexMetrics();
+            Assert.AreEqual(
+                null,
+                indexMetrics);
+        }
+
         [DataRow(false)]
         [DataRow(true)]
         [TestMethod]