Add a snippet about header propogation

Author: pranavkm

aspnetcore/fundamentals/http-requests.md

@@ -957,6 +957,43 @@ In the following example:
 
 [!code-csharp[](http-requests/samples/2.x/HttpClientFactoryConsoleSample/Program.cs?highlight=14-15,20,26-27,59-62)]
 
+## Header propogation middleware
+
+Header propogation is an ASP.NET Core middleware to propagate HTTP headers from the incoming request to the outgoing HTTP Client requests. To use it, 
+
+* Reference the [`Microsoft.AspNetCore.HeaderPropagation`](https://www.nuget.org/packages/Microsoft.AspNetCore.HeaderPropagation) package. For projects targeting .NET Core 2.1, consider using the community supported port of the package - [`HeaderPropogation`](https://www.nuget.org/packages/HeaderPropagation).
+
+* Configure the middleware and HttpClient in your Startup:
+
+```C#
+// Startup.cs
+public void ConfigureServices(IServiceCollection services)
+{
+    services.AddHeaderPropagation(options =>
+    {
+        options.Headers.Add("Trace-Id");
+    }
+    
+    services.AddHttpClient("MyForwardingClient")
+       .AddHeaderPropogation();
+}
+
+public void Configure(IApplicationBuilder app)
+{
+    app.UseHeaderPropogation();
+    app.UseRouting();
+    ...
+}
+```
+
+* The client will include the configured headers on outbound requests:
+
+```C#
+var client = clientFactory.CreateClient("MyForwardingClient");
+var response = client.GetAsync(...);
+```
+
+
 ## Additional resources
 
 * [Use HttpClientFactory to implement resilient HTTP requests](/dotnet/standard/microservices-architecture/implement-resilient-applications/use-httpclientfactory-to-implement-resilient-http-requests)