update samples.

samples/Sample.RabbitMQ.MySql/Startup.cs

-using Microsoft.AspNetCore.Builder;
+using System;
+using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Hosting;
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Logging;
@@ -16,6 +17,11 @@ namespace Sample.RabbitMQ.MySql
                 x.UseEntityFramework<AppDbContext>();
                 x.UseRabbitMQ("localhost");
                 x.UseDashboard();
+                x.FailedRetryCount = 5;
+                x.FailedThresholdCallback = (type, name, content) =>
+                {
+                    Console.WriteLine($@"A message of type {type} failed after executing {x.FailedRetryCount} several times, requiring manual troubleshooting. Message name: {name}, message body: {content}");
+                };
             });
 
             services.AddMvc();