Add microbenchmark for core UnmanagedMemoryStream scenario

src/benchmarks/micro/libraries/System.IO/UnmanagedMemoryStreamTests.cs

@@ -0,0 +1,53 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System.IO;
+using System.Runtime.InteropServices;
+using BenchmarkDotNet.Attributes;
+using MicroBenchmarks;
+
+namespace System.IO.Tests
+{
+    [BenchmarkCategory(Categories.Libraries)]
+    public unsafe class UnmanagedMemoryStreamTests
+    {
+        private const int Length = 10000;
+        private byte* _buffer;
+
+        [GlobalSetup]
+        public void Setup()
+        {
+            _buffer = (byte*)Marshal.AllocCoTaskMem(Length);
+        }
+
+        [GlobalCleanup]
+        public void Cleanup()
+        {
+            Marshal.FreeCoTaskMem((IntPtr)_buffer);
+        }
+
+        [Benchmark]
+        public void ReadAsBytes()
+        {
+            using (var ums = new UnmanagedMemoryStream(_buffer, Length))
+            {
+                while (ums.ReadByte() >= 0)
+                {
+                }
+            }
+        }
+
+        [Benchmark]
+        public void ReadAsArrays()
+        {
+            var array = new byte[128];
+            using (var ums = new UnmanagedMemoryStream(_buffer, Length))
+            {
+                while (ums.Read(array, 0, array.Length) != 0)
+                {
+                }
+            }
+        }
+    }
+}