ASP.NET Core 10 Adds Built-In Authentication Metrics to Distinguish Auth Outcomes

ASP.NET Core 10 introduces built-in authentication and authorization instruments via System.Diagnostics.Metrics, allowing developers to differentiate between three distinct handler outcomes: no credentials present, credentials rejected, and successful authentication. Previously, both unauthenticated and rejected requests would surface identically as 401 responses, making root-cause diagnosis difficult. The new aspnetcore.authentication.authenticate.duration histogram exposes an aspnetcore.authentication.result attribute that labels each outcome, while a separate aspnetcore.authentication.challenges counter tracks how often a scheme was challenged. Developers can integrate these metrics into an OpenTelemetry pipeline by registering the Microsoft.AspNetCore.Authentication meter, though a separate exporter and backend are still needed for production storage. The framework also attaches an error.type attribute on failures, deliberately using exception types rather than messages to avoid leaking sensitive data such as tokens or user identifiers into metric labels.