1. 首先，在ASP.NET Core 6.0应用程序项目中添加以下NuGet软件包：Microsoft.AspNetCore.Mvc.Core和Microsoft.AspNetCore.Mvc.NewtonsoftJson。

2. 在Startup.cs文件中添加以下代码：

using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Newtonsoft.Json.Serialization;

namespace CountingApp
{
    public class Startup
    {
        public IConfiguration Configuration { get; }

        public Startup(IConfiguration configuration)
        {
            Configuration = configuration;
        }

        public void ConfigureServices(IServiceCollection services)
        {
            services.AddControllersWithViews()
                .AddNewtonsoftJson(options =>
                {
                    options.SerializerSettings.ContractResolver = new DefaultContractResolver
                    {
                        NamingStrategy = new SnakeCaseNamingStrategy()
                    };
                });
        }

        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }

            app.UseRouting();

            app.UseEndpoints(endpoints =>
            {
                endpoints.MapControllers();
            });
        }
    }
}

3. 在Controllers文件夹中添加一个名为"CountingController.cs"的控制器：

using Microsoft.AspNetCore.Mvc;

namespace CountingApp.Controllers
{
    [ApiController]
    [Route("[controller]")]
    public class CountingController : ControllerBase
    {
        [HttpGet("{num}")]
        public IActionResult Get(string num)
        {
            int count = num.Length;
            return Ok(new { count });
        }
    }
}

4. 启动应用程序并使用以下命令进行测试：

https://localhost:5001/counting/12345

这将返回以下结果：

{
    "count": 5
}