For every metric you track, define a counter-metric. If you optimise for sign-ups, track activation too. Show you guard against unintended consequences.
Strong answers demonstrate understanding of Goodhart's Law: when a measure becomes a target, it ceases to be a good measure. Look for candidates who think about leading vs lagging indicators, counter-metrics to prevent gaming, and metric hierarchies. Best candidates have concrete examples of replacing vanity metrics with actionable ones.
Tests analytical maturity. PMs who pick metrics without considering second-order effects create perverse incentives. Ask follow-up: "How do you get a team to care about a metric they did not choose?"