But I'm stuck on the Processor how to add something to the existing method

I am not 100% sure but as I recall preprocessor should not modify existing code. Tools like mockito/autovalue generate subclass, override method do some additional operation there and call super method

I ended up solving it in a different way, without annotation, I created a custom function

fun myFun(function: () -> Any) { .. }

and just wrapped the method bodies inside it

Yep, that's an easy way with higher order function and inside higher order function have invoke method inside measureTimeInMillis and inline this higher order function.