UseAppFiltersUseCase.cs

using Microsoft.Extensions.Logging;
using Microsoft.Extensions.Logging.EventSource;
using System;
using System.Collections.Generic;
using System.Diagnostics.Tracing;
using System.Threading;
using System.Threading.Tasks;
namespace ConsoleApp46
{
    class Program
    {
        public static async Task Main(string[] args)
        {
            MyEventListener listener = new MyEventListener();
            using var loggerFactory = LoggerFactory.Create(builder =>
            {
                builder
                    .AddEventSourceLogger()
                    //.AddConsole()
                    .AddFilter("Component1*", LogLevel.Warning)
                    .AddFilter("Component2*", LogLevel.Error)
                    ;
            });
            var logger1 = loggerFactory.CreateLogger("Component11");
            var logger2 = loggerFactory.CreateLogger("Component22");
            var logger3 = loggerFactory.CreateLogger("Component33");
            while (true)
            {
                logger1.LogCritical("LogCritical1");
                logger1.LogDebug("LogDebug1");
                logger1.LogError("LogError1");
                logger1.LogInformation("LogInformation1");
                logger1.LogTrace("LogTrace1");
                logger1.LogWarning("LogWarning1");

                logger2.LogCritical("LogCritical2");
                logger2.LogDebug("LogDebug2");
                logger2.LogError("LogError2");
                logger2.LogInformation("LogInformation2");
                logger2.LogTrace("LogTrace2");
                logger2.LogWarning("LogWarning2");

                logger3.LogCritical("LogCritical3");
                logger3.LogDebug("LogDebug3");
                logger3.LogError("LogError3");
                logger3.LogInformation("LogInformation3");
                logger3.LogTrace("LogTrace3");
                logger3.LogWarning("LogWarning3");

                Thread.Sleep(1000);
            }
        }
    }
    class MyEventListener : EventListener
    {
        protected override void OnEventSourceCreated(EventSource eventSource)
        {
            if (eventSource.Name == "Microsoft-Extensions-Logging")
            {
                EnableEvents(eventSource, EventLevel.Verbose, (EventKeywords)0x4
                    , new Dictionary<string, string?>()
                {
                    { "FilterSpecs", "Component33:5" }
                });
            }
        }
        protected override void OnEventWritten(EventWrittenEventArgs eventData)
        {
            Console.WriteLine("MEL Event: " + eventData.EventName + " " + eventData.Level);
            for (int i = 0; i < eventData.Payload.Count; i++)
            {
                Console.WriteLine("    " + eventData.PayloadNames[i] + " = " + eventData.Payload[i]);
            }
        }
    }
}

Output before PR:

MEL Event: FormattedMessage LogAlways
    Level = 5
    FactoryID = 1
    LoggerName = Component33
    EventId = 0
    EventName =
    FormattedMessage = LogCritical3

After PR:
Using "Component33:5" as FilterSpec the output is the same.
But when using "UseAppFilters;Component33:5" as the FilterSpec we get:

MEL Event: FormattedMessage LogAlways
    Level = 5
    FactoryID = 1
    LoggerName = Component11
    EventId = 0
    EventName =
    FormattedMessage = LogCritical1
MEL Event: FormattedMessage LogAlways
    Level = 4
    FactoryID = 1
    LoggerName = Component11
    EventId = 0
    EventName =
    FormattedMessage = LogError1
MEL Event: FormattedMessage LogAlways
    Level = 3
    FactoryID = 1
    LoggerName = Component11
    EventId = 0
    EventName =
    FormattedMessage = LogWarning1
MEL Event: FormattedMessage LogAlways
    Level = 5
    FactoryID = 1
    LoggerName = Component22
    EventId = 0
    EventName =
    FormattedMessage = LogCritical2
MEL Event: FormattedMessage LogAlways
    Level = 4
    FactoryID = 1
    LoggerName = Component22
    EventId = 0
    EventName =
    FormattedMessage = LogError2
MEL Event: FormattedMessage LogAlways
    Level = 5
    FactoryID = 1
    LoggerName = Component33
    EventId = 0
    EventName =
    FormattedMessage = LogCritical3