[DOCS] Better / improved instructions for APM Integration monitoring for on-premise

[lucabelluccini]

The page https://www.elastic.co/guide/en/observability/current/apm-monitor-apm-self-install.html#apm-configure-ea-monitoring-data

1. We mention one can edit elastic-agent.yml and enroll Elastic Agent. This option is valid only if one has not yet enrolled Elastic Agent.
   If ones does so when they have already an enrolled agent, they might likely lose the state of Elastic Agent.

=> We need a banner/warning

2. Since 8.15.0 onwards, Fleet UI is able to change the HTTP monitoring port of EA running a specific policy (https://www.elastic.co/guide/en/fleet/8.15/agent-policy.html#agent-policy-http-monitoring)

=> I think this can be added to the documentation as it's way better for already enrolled agents (TO BE TESTED FIRST)

3. The documentation mentions Metricbeat Beat X-Pack Module, including username and password, but there's no need here

=> I would remove it and add a banner mentioning the endpoint is not protected.

[lucabelluccini]

Hello @colleenmcginnis - IDK if this should be in the observability-docs or here.
You might need engineering review/input on one point above.

[colleenmcginnis]

Thanks @lucabelluccini! (I'll add it to the observability docs board. We'll triage it there and decide whether or not to move it to observability-docs.)