[APM] Plugin & request context and query setup

[dgieselaar]

In #49455, we've moved closer to the new platform, and made some changes to how requests are handled. Request handlers now get executed with the KibanaRequest object, the APMRequestContext object (which is an extension of Kibana's RequestContextobject), and the legacyServerobject. After that, most of the endpoints will callsetupRequestwithrequestandcontext(returning aSetup` object), to bootstrap some things, like getting the APM indices

There are several changes in NP that might affect this process:

• Exposing the internal repository for saved objects in NP: Split legacy plugin discovery, expose SavedObjects scopedClient, wrappers, repository #48882
• migrate usage collector to NP: collectorSets in the New Platform #46924
• removing data/admin client distinction for the ES client: Core Elasticsearch: remove "data" and "admin" clients and expose single client #49870

After this work has been completed we could consider the ramifications of our current abstractions, and see where we could possibly improve things. Right now there's some overlap between APMRequestContext and Setup, and in some cases we have to pass both objects around.

[elasticmachine]

Pinging @elastic/apm-ui (Team:apm)

[sorenlouv]

@dgieselaar Any updates on this now that NP migration is done?

[sorenlouv]

@dgieselaar Can this be closed?

[dgieselaar]

@sqren hmm, yeah, not sure if we did exactly as proposed but APMRequestContext does not exist anymore.

[sorenlouv]

Yeah, I'm not sure either. I'll close and then we can open another issue if needed.