Add missing snippet to sesson doc (#17269)

* Add missing snippet to sesson doc

* Add missing snippet to sesson doc

aspnetcore/fundamentals/app-state.md

@@ -89,7 +89,7 @@ To enable the session middleware, `Startup` must contain:
 
 The following code shows how to set up the in-memory session provider with a default in-memory implementation of `IDistributedCache`:
 
-[!code-csharp[](app-state/samples/3.x/SessionSample/Startup.cs?name=snippet1&highlight=12-19,39)]
+[!code-csharp[](app-state/samples/3.x/SessionSample/Startup4.cs?name=snippet1&highlight=12-19,39)]
 
 The preceding code sets a short timeout to simplify testing.
 

aspnetcore/fundamentals/app-state/samples/3.x/SessionSample/Extensions/SessionExtensions.cs

@@ -17,11 +17,14 @@ namespace Web.Extensions
             return value == null ? default : JsonSerializer.Deserialize<T>(value);
         }
     }
-    #endregion
+    #endregion      
+}
 
+namespace Web.Extensions2
+{
     // Alternate approach
 
-    public static class SessionExtensions2
+    public static class SessionExtensions
     {
         public static void Set<T>(this ISession session, string key, T value)
         {

aspnetcore/fundamentals/app-state/samples/3.x/SessionSample/Startup4.cs

@@ -0,0 +1,67 @@
+using Microsoft.AspNetCore.Builder;
+using Microsoft.AspNetCore.Hosting;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Hosting;
+using SessionSample.Middleware;
+using System;
+
+namespace SessionSample2
+{
+    #region snippet1
+    public class Startup
+    {
+        public Startup(IConfiguration configuration)
+        {
+            Configuration = configuration;
+        }
+
+        public IConfiguration Configuration { get; }
+
+        public void ConfigureServices(IServiceCollection services)
+        {
+            services.AddDistributedMemoryCache();
+
+            services.AddSession(options =>
+            {
+                options.IdleTimeout = TimeSpan.FromSeconds(10);
+                options.Cookie.HttpOnly = true;
+                options.Cookie.IsEssential = true;
+            });
+
+            services.AddControllersWithViews();
+            services.AddRazorPages();
+        }
+
+        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
+        {
+            if (env.IsDevelopment())
+            {
+                app.UseDeveloperExceptionPage();
+            }
+            else
+            {
+                app.UseExceptionHandler("/Home/Error");
+                app.UseHsts();
+            }
+            app.UseHttpsRedirection();
+            app.UseStaticFiles();
+
+            app.UseHttpContextItemsMiddleware();
+
+            app.UseRouting();
+
+            app.UseAuthentication();
+            app.UseAuthorization();
+
+            app.UseSession();
+
+            app.UseEndpoints(endpoints =>
+            {
+                endpoints.MapDefaultControllerRoute();
+                endpoints.MapRazorPages();
+            });
+        }
+    }
+    #endregion
+}