Add a snippet about header propogation (dotnet#16705)

* Add a snippet about header propogation

Co-authored-by: Rick Anderson <[email protected]>

Author: pranavkm

aspnetcore/fundamentals/http-requests.md

@@ -347,6 +347,22 @@ In the following example:
 
 [!code-csharp[](http-requests/samples/3.x/HttpClientFactoryConsoleSample/Program.cs?highlight=14-15,20,26-27,59-62)]
 
+## Header propagation middleware
+
+Header propagation is an ASP.NET Core middleware to propagate HTTP headers from the incoming request to the outgoing HTTP Client requests. To use header propagation:
+
+* Reference the [Microsoft.AspNetCore.HeaderPropagation](https://www.nuget.org/packages/Microsoft.AspNetCore.HeaderPropagation) package.
+* Configure the middleware and `HttpClient` in `Startup`:
+
+  [!code-csharp[](http-requests/samples/3.x/Startup.cs?highlight=5-9,21&name=snippet)]
+
+* The client includes 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)
@@ -957,6 +973,23 @@ In the following example:
 
 [!code-csharp[](http-requests/samples/2.x/HttpClientFactoryConsoleSample/Program.cs?highlight=14-15,20,26-27,59-62)]
 
+## Header propagation middleware
+
+Header propagation is a community supported middleware to propagate HTTP headers from the incoming request to the outgoing HTTP Client requests. To use header propagation:
+
+* Reference the community supported port of the package [HeaderPropagation](https://www.nuget.org/packages/HeaderPropagation). ASP.NET Core 3.1 and later supports [Microsoft.AspNetCore.HeaderPropagation](https://www.nuget.org/packages/Microsoft.AspNetCore.HeaderPropagation).
+
+* Configure the middleware and `HttpClient` in `Startup`:
+
+  [!code-csharp[](http-requests/samples/2.x/Startup21.cs?highlight=5-9,25&name=snippet)]
+
+* The client includes 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)

aspnetcore/fundamentals/http-requests/samples/2.x/Startup21.cs

@@ -0,0 +1,49 @@
+using Microsoft.AspNetCore.Builder;
+using Microsoft.AspNetCore.Hosting;
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace HeaderProp21
+{
+    public class Startup21
+    {
+        public Startup21(IConfiguration configuration)
+        {
+            Configuration = configuration;
+        }
+
+        public IConfiguration Configuration { get; }
+
+        #region snippet
+        public void ConfigureServices(IServiceCollection services)
+        {
+            services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_1);
+
+            services.AddHttpClient("MyForwardingClient").AddHeaderPropagation();
+            services.AddHeaderPropagation(options =>
+            {
+                options.Headers.Add("X-TraceId");
+            });
+        }
+
+        public void Configure(IApplicationBuilder app, IHostingEnvironment env)
+        {
+            if (env.IsDevelopment())
+            {
+                app.UseDeveloperExceptionPage();
+            }
+            else
+            {
+                app.UseHsts();
+            }
+
+            app.UseHttpsRedirection();
+
+            app.UseHeaderPropagation();
+
+            app.UseMvc();
+        }
+        #endregion
+    }
+}

aspnetcore/fundamentals/http-requests/samples/3.x/Startup.cs

@@ -0,0 +1,52 @@
+using Microsoft.AspNetCore.Builder;
+using Microsoft.AspNetCore.Hosting;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Hosting;
+
+namespace HeaderProp
+{
+    public class Startup
+    {
+        public Startup(IConfiguration configuration)
+        {
+            Configuration = configuration;
+        }
+
+        public IConfiguration Configuration { get; }
+
+        #region snippet
+        public void ConfigureServices(IServiceCollection services)
+        {
+            services.AddControllers();
+
+            services.AddHttpClient("MyForwardingClient").AddHeaderPropagation();
+            services.AddHeaderPropagation(options =>
+            {
+                options.Headers.Add("X-TraceId");
+            });
+        }
+
+        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
+        {
+            if (env.IsDevelopment())
+            {
+                app.UseDeveloperExceptionPage();
+            }
+
+            app.UseHttpsRedirection();
+
+            app.UseHeaderPropagation();
+
+            app.UseRouting();
+
+            app.UseAuthorization();
+
+            app.UseEndpoints(endpoints =>
+            {
+                endpoints.MapControllers();
+            });
+        }
+        #endregion
+    }
+}