Fix cgroup helper init in add_processor_metadata and add_docker_metadata processors

[fearful-symmetry]

Proposed commit message

Closes #41065

This refactors add_processor_metadata and add_docker_metadata so we only call NewReader once when we create a new processor, and not every time the processor runs. This should considerably cut down on CPU cycles for certain workloads, since NewReader has gotten much more complex over the years.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• 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.

[elasticmachine]

Pinging @elastic/sec-linux-platform (Team:Security-Linux Platform)

[mergify]

This pull request does not have a backport label.
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[mergify]

backport-8.x has been added to help with the transition to the new branch 8.x.
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[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[belimawr]

Not my area of expertise, but LGTM.

I've seen the same CI error transaction_log_integration_test.go:22: timeout waiting for services to be healthy in one of my PRs, so I believe it's related to this one.

[rockdaboot]

@fearful-symmetry Thank you so much!

[Kavindu-Dodan]

This PR seems to break MAC os development of beats if any of the metadata processors are enabled. In my setup, I get Exiting: error initializing processors: error creating cgroup reader: cgroups not found or unsupported by OS [1]

[1] - https://github.com/elastic/beats/pull/41108/files#diff-5fb40da34716c19b3759ec0b54acbafe4f88a27fdf822593b56683343d35089eR166-R169