Every request to and from the app should be instrumented. Paying addition to the requests to the app is a good start - but you really need detailed instrumentation of all downstream dependencies your service uses to process it's requests to understand where the issue is. It's often likely you're slow or throwing errors because a dependency you use is slow or throwing errors. Or maybe the upstream service complaining has changed it's request pattern and they're making more expensive queries. There is often a small minority of requests that are responsible for most of the performance issues so even if the overall volume hasn't changed, the composition and type of requests matter as well.