Use optional values for docker memory metrics

[fearful-symmetry]

Proposed commit message

Closes #40060
This deals with an issue where we were reporting docker memory rss metrics even though none exist under cgroups V2. This takes the easy way out by just changing the hashmaps to use option types, so we don't report 0 values when none should exist.

I'm not sure there's a better way to do this. As far as I can tell, there's no similar metrics in Docker's cgroups V2 reporting, and in my experience trying to be clever and derive a substitute value usually backfires for heterogeneous monitoring setups.

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.

Disruptive User Impact

In cases where the docker/memory metrics were reporting an invalid 0, they will not not report any value at all.

[elasticmachine]

Pinging @elastic/security-external-integrations (Team:Security-External Integrations)

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @fearful-symmetry? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
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To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit

[mergify]

backport-8.x has been added to help with the transition to the new branch 8.x.
If you don't need it please use backport-skip label and remove the backport-8.x label.

[elasticmachine]

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

[mauri870]

LGTM