I have to improve performance of spring boot app, which is quite classical rest API + hibernate + postgres. we have 250k active users and want to extract some requests to be on slave balanced instances, and probably cache some data. For now i have only suspect that some requests need to be cached, But i want to make some audit and report, that some request called so many times that we should use other strategy, or "this" sql request fired every rest call so it's eat a lot DB lifetime which could be worked out using cache. Is there any best practice to make this kind of audit/analytic? Request-rate, request rate per user, SQL rate per request, SQL rate per user per request, and some other metrics
Spring Boot's metrics should give you a good starting point. The Spring MVC metrics should allow you to identify if there are certain types of request that are taking longer than others. Depending on how you are accessing your database, there are also DataSource metrics, Hibernate metrics, and Spring Data Repository metrics (new in Spring Boot 2.5) that may be of interest.
These metrics will be for your application as a whole rather than per-user. With over 250k active users, tagging metrics on a per-user basis almost certainly won't be practical. Unless you suspect that there are specific users that are problematic, I would at least start with the application-wide metrics and see how things go.