Remove .NET 10 Identity metrics workaround

The workaround for manually registering UserManagerMetrics and
SignInManagerMetrics types in the DI container is no longer needed
as the fix has been included in .NET 10.0.1+. The upstream ASP.NET
Core issue (dotnet/aspnetcore#64271) has been resolved and the
metrics classes are now instantiated directly without requiring DI
registration.

Changes:
- Removed manual registration code for UserManagerMetrics type
- Removed unused using statements (System.Diagnostics.Metrics and
  Microsoft.Extensions.DependencyInjection.Extensions)
- Updated CHANGELOG.md to document the removal

Fixes #4623

Author: aviralgarg05

test/test-applications/integrations/TestApplication.Http/Startup.cs

@@ -7,9 +7,7 @@
 using Microsoft.AspNetCore.Authentication.Cookies;
 using Microsoft.AspNetCore.Authorization;
 #if NET10_0_OR_GREATER
-using System.Diagnostics.Metrics;
 using Microsoft.AspNetCore.Identity;
-using Microsoft.Extensions.DependencyInjection.Extensions;
 #endif
 
 namespace TestApplication.Http;
@@ -54,26 +52,6 @@ public static void ConfigureServices(IServiceCollection services)
         services.AddIdentityCore<TestUser>()
             .AddUserManager<UserManager<TestUser>>();
 
-        // Workaround for .NET 10 bug: Manually register the internal UserManagerMetrics type
-        // Remove this workaround when upgrading to a .NET version that contains the fix
-        // Tracked under https://github.com/open-telemetry/opentelemetry-dotnet-instrumentation/issues/4623
-        var userManagerMetricsType = typeof(UserManager<>).Assembly.GetType("Microsoft.AspNetCore.Identity.UserManagerMetrics");
-        if (userManagerMetricsType != null)
-        {
-            services.TryAddSingleton(
-                userManagerMetricsType,
-                serviceProvider =>
-                {
-                    var meterFactory = serviceProvider.GetRequiredService<IMeterFactory>();
-                    return Activator.CreateInstance(
-                        userManagerMetricsType,
-                        System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.Public | System.Reflection.BindingFlags.NonPublic,
-                        null,
-                        [meterFactory],
-                        null)!;
-                });
-        }
-
         // Add in-memory user store for testing
         services.AddSingleton<IUserStore<TestUser>, InMemoryUserStore>();
 #endif