Which approach delivers real time alerts for Cloud Spanner transaction failures and also reflects overall application health?

You are designing an augmented reality game for iOS and Android devices. You plan to use Cloud Spanner as the primary backend database for game state storage and player authentication. You want to track in-game rewards that players unlock at every stage of the game. During the testing phase, you discovered that costs are much higher than anticipated, but the query response times are within the SLA. You want to follow Google-recommended practices.
You need the database to be performant and highly available while you keep costs low. What should you do?