To measure application reliability from a user perspective without making any engineering changes, the best approaches are to create new synthetic clients to simulate a user journey (D) and use current and historic Request Logs to trace customer interaction (E). Synthetic clients (D) allow you to simulate real user interactions without altering the application, providing insights into reliability from a user's perspective. Analyzing current and historic Request Logs (E) gives you a passive way to understand how users are interacting with your application and identify any issues they might be facing, all without the need for code changes. Options A and B involve making changes to the application or its monitoring setup, which contradicts the requirement of not making engineering changes. Option C is limited as it only considers the response time from web proxy logs, which doesn't fully capture the user's experience or reliability.