Describe the enhancement:

APM tracing config should additionally include a section for trace sampling to enable APM Server monitoring be configured with custom sampling rate when managed by Elastic Agent.

Describe a specific use case for the enhancement or feature:

Allow to specify a custom trace sampling rate to control the ingestion rate when self monitoring running APM Server.

What is the definition of done?

The custom sampling APM config section is handled correctly and used to configure tracing in APM Server.