A retail company runs a fleet of stateless microservices on Cloud Run behind an external HTTPS load balancer. During peak sale events, the operations team notices that a small percentage of user requests intermittently fail with elevated latency, but standard dashboards show all services as healthy and within their SLOs. Leadership wants a way to continuously verify the complete end-to-end user journey (login, browse, add-to-cart, checkout) from an external perspective and be alerted the moment any step of that journey degrades, even when individual service metrics look normal. Which approach best meets this requirement?