Design an algorithm to rank ads in the Google Play Store. Include primary and secondary success metrics.

Focus on defining the key factors that would influence ad rankings in the Google Play Store, such as relevance to user queries, click-through rates, and conversion rates. Consider how user engagement and app performance metrics could play a role in the algorithm. For primary success metrics, prioritize metrics like ad click-through rate (CTR) and return on ad spend (ROAS). For secondary metrics, think about user retention rates and app download rates post-ad click. Be prepared to justify your choices and explain how these metrics align with overall business goals.