You support a high-traffic web application that runs on Google Cloud Platform (GCP). You need to measure application reliability from a user perspective without making any engineering changes to it…

While reviewing current metrics is a good first step, 'adding new ones as needed' often implies needing to modify the application or infrastructure to emit those new metrics, which contradicts the 'without making any engineering changes' constraint.

Modifying the code to capture additional information directly violates the 'without making any engineering changes' constraint.

Analyzing *only* web proxy logs might be limiting and doesn't explicitly capture the full 'user perspective' as comprehensively as synthetic clients for active checks, or full request logs for passive checks, which are often more readily available.

Synthetic clients simulate actual user journeys by making requests to the application from external locations, providing an objective measure of availability, latency, and functionality from an end-user perspective without requiring any modifications to the application code. This is a form of active monitoring.

Request logs (e.g., from HTTP(S) Load Balancers, Cloud Logging) contain valuable information about real user interactions, including response times, status codes, and user agents. Analyzing these logs passively allows you to derive reliability metrics from actual user traffic without any code changes to the application itself.