Define success metrics beyond time spent

For a calling product, define success metrics for a feature launch. Critique "time spent" as a primary metric and propose a better primary metric set (with precise definitions) and guardrails. Then define retention properly: give cohort-based formulas for day-1, week-1, and rolling 28-day retention (active_if = placed_or_received_call OR sent_message). Specify numerator/denominator, cohorting date, and whether to include resurrected users. Finally, outline how you would monitor for negative side effects (e.g., call failure rate, complaint rate) and how you'd combine metrics into a decision rubric.