Add code sample from source release notes to rate-limiting include file

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Co-authored-by: wadepickett <10985336+wadepickett@users.noreply.github.com>

Author: Copilot

aspnetcore/release-notes/aspnetcore-11/includes/rate-limiting-retry-after-preview4.md

@@ -4,6 +4,25 @@ The <xref:System.Threading.RateLimiting.FixedWindowRateLimiter> now reports a <x
 
 Previously, the `RetryAfter` metadata wasn't reliably set, which meant rejected responses either lacked the header or contained an unhelpful value. With this fix, any app that uses the rate-limiting middleware with a <xref:System.Threading.RateLimiting.FixedWindowRateLimiter> benefits from correct `Retry-After` headers automatically.
 
+The following example shows how a rate limiter lease exposes the `RetryAfter` metadata after a failed acquisition:
+
+```csharp
+using System.Threading.RateLimiting;
+
+var limiter = new FixedWindowRateLimiter(new()
+{
+    PermitLimit = 10,
+    Window = TimeSpan.FromSeconds(1),
+    QueueLimit = 0,
+});
+
+RateLimitLease lease = limiter.AttemptAcquire();
+if (!lease.IsAcquired && lease.TryGetMetadata(MetadataName.RetryAfter, out TimeSpan retry))
+{
+    Console.WriteLine($"Retry after {retry}.");
+}
+```
+
 Additional fixes in `System.Threading.RateLimiting` resolve an issue where <xref:System.Threading.RateLimiting.TokenBucketRateLimiter> mishandled partial token refills during zero-permit acquisition, and improve <xref:System.Threading.RateLimiting.ChainedRateLimiter%601> to correctly forward idle-duration and replenishment behavior from its inner limiters.
 
 For more information, see [Rate limiting middleware in ASP.NET Core](/aspnet/core/performance/rate-limit).