Update httpclient-guidelines.md (#36866)

* Update httpclient-guidelines.md

Added example of using a Polly policy with a static HttpClient instance.

* Update docs/fundamentals/networking/http/httpclient-guidelines.md

Co-authored-by: David Pine <[email protected]>

* Update docs/fundamentals/networking/http/httpclient-guidelines.md

Co-authored-by: David Pine <[email protected]>

* Apply suggestions from code review

---------

Co-authored-by: David Pine <[email protected]>

Author: dimitriskad

docs/fundamentals/networking/http/httpclient-guidelines.md

@@ -52,6 +52,38 @@ To summarize recommended `HttpClient` use in terms of lifetime management, you s
 
 For more information about managing `HttpClient` lifetime with `IHttpClientFactory`, see [`IHttpClientFactory` guidelines](../../../core/extensions/httpclient-factory.md#httpclient-lifetime-management).
 
+## Resilience policies with static clients
+
+It's possible to configure a `static` or *singleton* client to use any number of resilience policies using the following pattern:
+
+```csharp
+using System;
+using System.Net.Http;
+using Microsoft.Extensions.Http;
+using Polly;
+using Polly.Extensions.Http;
+
+var retryPolicy = HttpPolicyExtensions
+    .HandleTransientHttpError()
+    .WaitAndRetryAsync(3, retryAttempt => TimeSpan.FromSeconds(Math.Pow(2, retryAttempt)));
+
+var socketHandler = new SocketsHttpHandler { PooledConnectionLifetime = TimeSpan.FromMinutes(15) };
+var pollyHandler = new PolicyHttpMessageHandler(retryPolicy)
+{
+    InnerHandler = socketHandler;
+};
+
+var httpClient = new HttpClient(pollyHandler);
+```
+
+The preceding code:
+
+- Relies on [Microsoft.Extensions.Http.Polly](https://www.nuget.org/packages/Microsoft.Extensions.Http.Polly) NuGet package, transitively the [Polly.Extensions.Http](https://www.nuget.org/packages/Polly.Extensions.Http) NuGet package for the `HttpPolicyExtensions` type.
+- Specifies a transient HTTP error handler, configured with retry policy that with each attempt will exponentially backoff delay intervals.
+- Defines a pooled connection lifetime of fifteen minutes for the `socketHandler`.
+- Passes the `socketHandler` to the `policyHandler` with the retry logic.
+- Instantiates an `HttpClient` given the `policyHandler`.
+
 ## See also
 
 - [HTTP support in .NET](http-overview.md)