chore: refactoring of apdex servicemetrics in preparation of adding an error counter apdex

chore: remove customApdex

This was unused

chore: move all apdex metrics into their own directory

This collects all of the apdex-type metrics into their own directory. We intend to add another one, which will make it a third one, and extract some of the shared parts into a library.

In this we also rename rateApdex to successCounterApdex. Allowing us to add an errorCounterApdex next to it.

chore: move resolveRateQuery for apdex into lib

This will allow us to reuse it for the error-counter-apdex.

chore: move apdexAttributionQuery into reusable lib

To be reused with an error counter query

Extracted from !5414 (merged) for gitlab-com/gl-infra/scalability#2204 (closed)