Fix flaky CounterSampleCalculator_ElapsedTime test

src/libraries/System.Diagnostics.PerformanceCounter/tests/CounterSampleCalculatorTests.cs

@@ -2,9 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System;
-using System.Collections;
-using System.Collections.Specialized;
 using Xunit;
+using Xunit.Sdk;
 
 namespace System.Diagnostics.Tests
 {
@@ -17,16 +16,27 @@ public static void CounterSampleCalculator_ElapsedTime()
 
             PerformanceCounter counterSample = CreateCounter(categoryName, PerformanceCounterType.ElapsedTime);
 
-            counterSample.RawValue = Stopwatch.GetTimestamp();
-            DateTime Start = DateTime.Now;
-            Helpers.RetryOnAllPlatforms(() => counterSample.NextValue());
-
-            System.Threading.Thread.Sleep(500);
-
-            var counterVal = Helpers.RetryOnAllPlatforms(() => counterSample.NextValue());
-            var dateTimeVal = DateTime.Now.Subtract(Start).TotalSeconds;
-            Helpers.DeleteCategory(categoryName);
-            Assert.True(Math.Abs(dateTimeVal - counterVal) < .3);
+            try
+            {
+                // Timing comparisons can be flaky under CI load, so retry.
+                RetryHelper.Execute(() =>
+                {
+                    long startTimestamp = Stopwatch.GetTimestamp();
+                    counterSample.RawValue = startTimestamp;
+                    Helpers.RetryOnAllPlatforms(() => counterSample.NextValue());
+
+                    System.Threading.Thread.Sleep(500);
+
+                    var counterVal = Helpers.RetryOnAllPlatforms(() => counterSample.NextValue());
+                    var elapsed = (double)(Stopwatch.GetTimestamp() - startTimestamp) / Stopwatch.Frequency;
+                    Assert.True(Math.Abs(elapsed - counterVal) < .3, $"Expected elapsed ({elapsed:F3}s) and counterVal ({counterVal:F3}s) to be within 0.3s");
+                }, maxAttempts: 3, retryWhen: e => e is XunitException);
+            }
+            finally
+            {
+                counterSample.Dispose();
+                Helpers.DeleteCategory(categoryName);
+            }
         }
 
         public static PerformanceCounter CreateCounter(string categoryName, PerformanceCounterType counterType)