database sharding strategy: when and how?

hey everyone, im thinking about database sharding for a growing application. we're hitting some performance bottlenecks with our single monolithic database, and sharding seems like the next logical step to improve scalability and availability. what are your go-to strategies when deciding *when* to shard and *how* to implement it? are there specific metrics you monitor that signal its time to shard? im curious about different approaches to shard key selection and potential pitfalls to avoid during the migration.