[APM] Make APM tracer configurable

[jalvz]

Describe the enhancement:

#17938 bring instrumentation to libbeat. However, the only way to configure the tracer is with environment variables. For a better experience, it would be ideal to configure it in the same way as other settings (via yaml, or -E args), and expose it (via Getter in the pipeline or similar) so that all beats can use it. This is particularly beneficial for APM Server and community Beats.

I therefore suggest a top level instrumentation namespace in the yaml config, initially with the following settings underneath:

• enabled: disabled by default
• environment: optional, typically prod, staging, etc - this info is reported by the agent and showed in Kibana
• hosts: apm server urls
• secret_token: for apm-server secure communication, optional
• api_key: for apm-server secure communication, optional
• profiling: experimental feature to report CPU and heap profiles, disabled by default

Describe a specific use case for the enhancement or feature:

The configuration proposed above is implemented already in APM Server. But every beat that wants a similar experience would need to create its own configuration and initialize the tracer accordingly.
It makes more sense to do it centrally and then all the beats would get it for free.

[jalvz]

@elastic/beats does this makes sense to you?

cc @elastic/apm-server