Guarded Rollout Align Percentile Metrics to Relative Difference Method

We're updating the method for detecting regression in percentile metrics. Previously, regression for percentile metrics was determined by checking whether intervals overlapped. With this update, we're moving to a relative difference-based approach, which can detect regressions that the interval overlap method might miss. This also brings percentile metrics in line with how mean metrics are evaluated, providing a more consistent experience in the UI.