Add System.Text.Json to the serialization tests for comparison.

Note: this doesn't full work. There is an exception in the runtime of these tests where the serialization of Dictionary<int, string> isn't a valid thing for System.Text.Json to handle as of 3.0 preview 7. The rest seems to work well and...I hope I added these correctly. We're trying to get an apples to apples comparison of serializers we can use in .NET Core for ASP.NET Core and async streams. We figured adding them here for everyone is the best course of action.

src/benchmarks/micro/Serializers/Json_FromStream.cs

@@ -108,6 +108,27 @@ public T DataContractJsonSerializer_()
             return (T)dataContractJsonSerializer.ReadObject(memoryStream);
         }
 
+#if NETCOREAPP3_0 // API Available in .NET Core 3.0+
+        [GlobalSetup(Target = nameof(SystemTextJson_))]
+        public void SetupSystemTextJson_()
+        {
+            memoryStream.Position = 0;
+
+            using (var writer = new System.Text.Json.Utf8JsonWriter(memoryStream))
+            {
+                System.Text.Json.JsonSerializer.Serialize<T>(writer, value);
+            }
+        }
+
+        [BenchmarkCategory(Categories.CoreFX, Categories.JSON)]
+        [Benchmark(Description = "System.Text.Json")]
+        public System.Threading.Tasks.ValueTask<T> SystemTextJson_()
+        {
+            memoryStream.Position = 0;
+            return System.Text.Json.JsonSerializer.DeserializeAsync<T>(memoryStream);
+        }
+#endif
+
         private StreamReader CreateNonClosingReaderWithDefaultSizes()
             => new StreamReader(
                 memoryStream, 

src/benchmarks/micro/Serializers/Json_FromString.cs

@@ -38,5 +38,14 @@ public class Json_FromString<T>
         [BenchmarkCategory(Categories.ThirdParty)]
         [Benchmark(Description = "Utf8Json")]
         public T Utf8Json_() => Utf8Json.JsonSerializer.Deserialize<T>(serialized);
+
+#if NETCOREAPP3_0 // API Available in .NET Core 3.0+
+        [GlobalSetup(Target = nameof(SystemTextJson_))]
+        public void SerializeSystemTextJson() => serialized = System.Text.Json.JsonSerializer.Serialize<T>(value);
+
+        [BenchmarkCategory(Categories.CoreFX, Categories.JSON)]
+        [Benchmark(Description = "System.Text.Json")]
+        public T SystemTextJson_() => System.Text.Json.JsonSerializer.Deserialize<T>(serialized);
+#endif
     }
 }

src/benchmarks/micro/Serializers/Json_ToStream.cs

@@ -68,6 +68,16 @@ public void DataContractJsonSerializer_()
             dataContractJsonSerializer.WriteObject(memoryStream, value);
         }
 
+#if NETCOREAPP3_0 // API Available in .NET Core 3.0+
+        [BenchmarkCategory(Categories.CoreFX, Categories.JSON)]
+        [Benchmark(Description = "System.Text.Json")]
+        public System.Threading.Tasks.Task SystemTextJson_()
+        {
+            memoryStream.Position = 0;
+            return System.Text.Json.JsonSerializer.SerializeAsync<T>(memoryStream, value);
+        }
+#endif
+
         [GlobalCleanup]
         public void Cleanup()
         {

src/benchmarks/micro/Serializers/Json_ToString.cs

@@ -31,5 +31,11 @@ public class Json_ToString<T>
 
         // DataContractJsonSerializer does not provide an API to serialize to string
         // so it's not included here (apples vs apples thing)
+
+#if NETCOREAPP3_0 // API Available in .NET Core 3.0+
+        [BenchmarkCategory(Categories.CoreFX, Categories.JSON)]
+        [Benchmark(Description = "System.Text.Json")]
+        public string SystemTextJson_() => System.Text.Json.JsonSerializer.Serialize<T>(value);
+#endif
     }
 }