Add LIBBEAT_MONITORING_CGROUPS_HIERARCHY_OVERRIDE to system/process

[fearful-symmetry]

What does this PR do?

So, a while ago @axw added support for LIBBEAT_MONITORING_CGROUPS_HIERARCHY_OVERRIDE in libbeat monitoring. However, it wasn't added to system/process. I'm going to assume it was just overlooked, and there wasn't a reason to exclude it, so this adds it, so we get proper cgroup metrics from a container.

Why is it important?

This is kind of a bug, as we properly report cgroup metrics within libbeat monitoring, but not within metricbeat.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

How to test this PR locally

• Pull down and build
• Run metricbeat inside a container, make sure cgroup metrics are correctly reported from within system/process

[elasticmachine]

Pinging @elastic/integrations (Team:Integrations)

[elasticmachine]

💚 Build Succeeded

the below badges are clickable and redirect to their specific view in the CI or DOCS

Expand to view the summary

Build stats

• Start Time: 2021-09-02T00:00:58.228+0000

• Duration: 84 min 41 sec

• Commit: 38de8fb

Test stats 🧪

Test	Results
Failed	0
Passed	9171
Skipped	2429
Total	11600

Trends 🧪

💚 Flaky test report

Tests succeeded.

Expand to view the summary

Test stats 🧪

Test	Results
Failed	0
Passed	9171
Skipped	2429
Total	11600

[axw]

LGTM! I originally only added for stack monitoring as that's all we needed, but I think it's sensible to apply it to regular system metrics too.