Add memory/cgroup fields to metricbeat Kibana module

[pgayvallet]

This PR updates the Metricbeat kibana module with support for the new memory and cgroup fields added to Kibana in elastic/kibana#172146. We plan to use this metric for serverless monitoring, so metricbeat needs to know about it.

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 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.

How to test this PR locally

• run kibana from source & ES from snapshot as explained in https://www.elastic.co/guide/en/kibana/master/development-getting-started.html
• pull down this PR, cd metricbeat
• mage build
• ./metricbeat modules enable kibana
• uncomment the elasticsearch.output username/password lines in metricbeat.yml (elastic:changeme)
• update ./modules.d/kibana.yml:

- module: kibana
  metricsets:
    - stats # be sure to enable stats metricset
  period: 10s
  hosts: ["localhost:5601"]
  basepath: "abc" # whatever your base path is
  username: "elastic"
  password: "changeme"

• ./metricbeat -e
• create metricbeat-* data view in kibana
• you should now be seeing some log entries which contain the new fields, eg. process.memory.array_buffers

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[pgayvallet]

run elasticsearch-ci/docs

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To re-run your PR in the CI, just comment with:

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[miltonhultgren]

My understanding was the Serverless uses Elastic Agent only and not Metricbeat directly?
If that's correct, could this not be added directly to the Kibana Integration instead and only there?
If it's needed for both Metricbeat and Elastic Agent, we'll need to also update the integrations mappings.

Further, will users on ESS or on-prem using the X-pack mode want these fields to be indexed as well?
In that case we need to update the mappings used for the X-pack mode as well.

[miltonhultgren]

The code changes themselves look correct