not use the extension to fix copilot comments

Author: WeihanLi

src/libraries/System.Threading.RateLimiting/src/System/Threading/RateLimiting/ConcurrencyLimiter.cs

@@ -32,7 +32,7 @@ public sealed class ConcurrencyLimiter : RateLimiter
         private object Lock => _queue;
 
         /// <inheritdoc />
-        public override TimeSpan? IdleDuration => Stopwatch.GetElapsedTime(_idleSince);
+        public override TimeSpan? IdleDuration => RateLimiterHelper.GetElapsedTime(_idleSince);
         /// <summary>
         /// Initializes the <see cref="ConcurrencyLimiter"/>.
         /// </summary>

src/libraries/System.Threading.RateLimiting/src/System/Threading/RateLimiting/FixedWindowRateLimiter.cs

@@ -24,10 +24,10 @@ public sealed class FixedWindowRateLimiter : ReplenishingRateLimiter
 
         /// <summary>
         /// Function to calculate elapsed time from a given tick value.
-        /// Defaults to the <c>Stopwatch.GetElapsedTime(long?)</c> extension which returns <see langword="null"/> when the timestamp is <see langword="null"/>.
+        /// Defaults to the <see cref="RateLimiterHelper.GetElapsedTime(long?)"/> which returns <see langword="null"/> when the timestamp is <see langword="null"/>, and returns <c>Stopwatch.GetElapsedTime(long)</c> when not null.
         /// In tests, this field can be reassigned via reflection to inject custom time behavior without modifying the public API.
         /// </summary>
-        private readonly Func<long?, TimeSpan?> _getElapsedTime = Stopwatch.GetElapsedTime;
+        private readonly Func<long?, TimeSpan?> _getElapsedTime = RateLimiterHelper.GetElapsedTime;
 
         private readonly Timer? _renewTimer;
         private readonly FixedWindowRateLimiterOptions _options;

src/libraries/System.Threading.RateLimiting/src/System/Threading/RateLimiting/RateLimiterHelper.cs

@@ -7,17 +7,14 @@ namespace System.Threading.RateLimiting
 {
     internal static class RateLimiterHelper
     {
-        extension(Stopwatch)
+        public static TimeSpan? GetElapsedTime(long? startTimestamp)
         {
-            public static TimeSpan? GetElapsedTime(long? startTimestamp)
+            if (startTimestamp is null)
             {
-                if (startTimestamp is null)
-                {
-                    return null;
-                }
-
-                return Stopwatch.GetElapsedTime(startTimestamp.Value);
+                return null;
             }
+
+            return Stopwatch.GetElapsedTime(startTimestamp.Value);
         }
     }
 }

src/libraries/System.Threading.RateLimiting/src/System/Threading/RateLimiting/SlidingWindowRateLimiter.cs

@@ -36,7 +36,7 @@ public sealed class SlidingWindowRateLimiter : ReplenishingRateLimiter
         private static readonly RateLimitLease FailedLease = new SlidingWindowLease(false, null);
 
         /// <inheritdoc />
-        public override TimeSpan? IdleDuration => Stopwatch.GetElapsedTime(_idleSince);
+        public override TimeSpan? IdleDuration => RateLimiterHelper.GetElapsedTime(_idleSince);
 
         /// <inheritdoc />
         public override bool IsAutoReplenishing => _options.AutoReplenishment;

src/libraries/System.Threading.RateLimiting/src/System/Threading/RateLimiting/TokenBucketRateLimiter.cs

@@ -34,7 +34,7 @@ public sealed class TokenBucketRateLimiter : ReplenishingRateLimiter
         private static readonly RateLimitLease FailedLease = new TokenBucketLease(false, null);
 
         /// <inheritdoc />
-        public override TimeSpan? IdleDuration => Stopwatch.GetElapsedTime(_idleSince);
+        public override TimeSpan? IdleDuration => RateLimiterHelper.GetElapsedTime(_idleSince);
 
         /// <inheritdoc />
         public override bool IsAutoReplenishing => _options.AutoReplenishment;