Wondering how to check API performance without going crazy?

Hey everyone, I'm trying to get a better handle on how to track our API performance, especially from the client's point of view. We're a growing startup, so I'm not looking for anything super complex like full APM suites just yet. What are some simple methods or metrics you've found useful for monitoring latency and error rates? Any easy-to-implement tools or techniques that give good insights without a huge setup cost? I'm curious to hear what others are doing.