Add support for optional metricsets in xpack mode

[joshdover]

What does this PR do?

This adds support for specifying that a metricset can also be enabled when xpack.enabled: true is set, but is not turned on by default. This allows for finer-grained control of when a metricset is enabled when using xpack mode. For example this will allow support for a configuration like:

- module: elasticsearch
  metricsets:
    - ingest_pipeline # turn on the optional ingest_pipeline metricset
  xpack.enabled: true # turn on all required xpack metricsets

This only affects the beat, elasticsearch, kibana, and logstash modules for Stack Monitoring.

Why is it important?

We are adding a new ingest metricset as "beta" in #34012. We want to be able to enable/disable this on Cloud deployments as we do testing. Since Cloud uses xpack.enabled: true we need a new way to have control over which metricsets are enabled.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
  • I have opted to skip this for now and will include docs in Add elasticsearch/ingest_pipeline metricset #34012 once there's actually a metricset using this new option
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Author's Checklist

• [ ]

How to test this PR locally

Not possible yet since there's no metricsets using this

Related issues

• Relates to Add elasticsearch/ingest_pipeline metricset #34012

[elasticmachine]

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• Start Time: 2023-01-24T14:30:58.139+0000

• Duration: 63 min 23 sec

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[joshdover]

@klacabane friendly bump here if you have some time to review

[ruflin]

Thanks for separating out this change. I'm good with moving forward with it. I think eventually we should extend it this logic to make it possible to also disable metricsets that are on by default but we can do this in a follow up.

[klacabane]

LGTM