GrandOutput is not configured yet in ConfigureServices()

[bcrosnier]

We've been using the ActivityMonitor in WebHostBuilder.ConfigureServices(), but any monitor we use there does not output to file.
Upon further inspection, this is basically caused by CK-AspNet's StartupFilter; StartupFilters are executed after the whole ConfigureServices() pipeline (which makes sense, since they're registered in the ConfigureServices() pipeline).

Example code:

using CK.Core;
using Microsoft.AspNetCore;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.DependencyInjection;
using System;

namespace WebApplication3
{
    public class Program
    {
        public static void Main(string[] args)
        {
            BuildWebHost(args).Run();
        }

        public static IWebHost BuildWebHost(string[] args) =>
            WebHost.CreateDefaultBuilder(args)
                .UseStartup<Startup>()
                .UseMonitoring()
                .ConfigureServices(services =>
                {
                    // GrandOutput doesn't have any handler yet
                    ActivityMonitor m = new ActivityMonitor();
                    m.Error("Error in ConfigureServices()");

                    // Register startup filter
                    services.AddSingleton<IStartupFilter, StartupFilterTest>();
                })
                .Configure((app) =>
                {
                    // Register ApplicationStarted
                    app.ApplicationServices.GetRequiredService<IApplicationLifetime>().ApplicationStarted.Register(OnApplicationStarted);
                })
                .Build();

        public class StartupFilterTest : IStartupFilter
        {
            public Action<IApplicationBuilder> Configure(Action<IApplicationBuilder> next) => (app) =>
            {
                // GrandOutput is configured
                ActivityMonitor m = new ActivityMonitor();
                m.Error("Error in StartupFilter");

                next?.Invoke(app);
            };
        }

        private static void OnApplicationStarted()
        {
            // GrandOutput is configured
            ActivityMonitor m = new ActivityMonitor();
            m.Error("Error in ApplicationStarted");
        }
    }
}