Database sharding: When do you start, and how do you do it?

been thinking bout database sharding. it feels like a big architectural step, and im curious bout when you all decide its time to pull the trigger. are there specific metrics or pain points that signal sharding is necessary? and once you decide, what are your preferred strategies for implementation? do you go with application-level sharding, proxy-based, or something else? keen to hear bout the trade-offs youve encountered