[APM] Agent Configuration test scenarios

[smith]

This is a list of recent issues and PRs related to Agent Configuration. All have been merged and have backports completed:

• .apm-agent-configuration is not created if Kibana is started while ES is not ready #59420 (PR [APM] .apm-agent-configuration is not created if Kibana is started while ES is not ready #61610)
• [APM] Update central configuration text #61556
• [APM] Agent remote config: Remove specific options from agents not supporting dynamic reloading #61821 (PR [7.x] removing configuration from agents (#62290) #62397)
• docs: Update APM central config documentation #61893
• [APM] Agent duration-type configuration input has incorrect/missing units #62110 (PR [APM] Agent duration-type configuration input has incorrect/missing units #62265)
• [APM] Agent remote configuration: general settings descriptions #62276
• [APM] Agent remote configuration: changes in Java property descriptions #62282
• [APM] Agent span_frames_min_duration configuration input cannot handle 0 or -1 #62569 (PR [APM] Agent span_frames_min_duration configuration input cannot handle 0 or -1 #62777)

Possibly related is elastic/apm-server#3579.

We need to make sure everything required for 7.7 is merged and backported, and everything not required is noted as such.

Most of these issues were raised by agent or other team members testing the feature, but we need to give it a good go-through once these are taken care of, as well as ensuring there's no browser-specific issues.

[elasticmachine]

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