CgroupCpuSetMonitor: Initialize the cgroup discoverer

[ac9817]

Description

Fixes the null pointer exceptions when the monitor is enabled.

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Key changed/added classes in this PR

• CgroupCpuSetMonitor

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This PR has:

• been self-reviewed.
  • using the concurrency checklist (Remove this item if the PR doesn't have any relation to concurrency.)
• added documentation for new or modified features or behaviors.
• a release note entry in the PR description.
• added Javadocs for most classes and all non-trivial methods. Linked related entities via Javadoc links.
• added or updated version, license, or notice information in licenses.yaml
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage is met.
• added integration tests.
• been tested in a test Druid cluster.

[suneet-s]

Overruling CI because it's failing on coverage

[kgyrtkirk]

fyi: there were 6 failing jobs ; one of them failed with injector issues:
https://github.com/apache/druid/actions/runs/9555883049/job/26342057882

the issue is rather strange - and it doesn't seem related at all to this PR; it could be some flaky issue?

I think it would be great to have a sacred way of disabling the coverage check for a PR - as it may hide legitim issues by creating noise