Enable self instrumentation in APM Server

[1pkg]

Motivation

Enable self instrumentation in APM Server.

Description

With the missing trace sampling added to self instrumentation configuration in PR it should be possible now to revert the changes that we did in the PR and enable self tracing in APM Server.

Related issues

• Add missing tracing sampling APM config section elastic-agent#5211
• fix: Temporary disable self trace apm config #13790
• feat: add reload listener for apm tracing config #13514