[Metricbeat] Add memory PSI metrics for cgroupv2

[orestisfl]

Proposed commit message

Add memory pressure PSI metrics to the system.process.cgroup.memory metricset, complementing the existing CPU and IO pressure metrics.

New fields added under system.process.cgroup.memory.pressure:

• pressure.some.{10,60,300}.pct - Share of time with some tasks stalled
• pressure.some.total - Total some pressure time
• pressure.full.{10,60,300}.pct - Share of time with all tasks stalled
• pressure.full.total - Total full pressure time

Closes #47604

Checklist

• [x] 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. Where relevant, I have used the stresstest.sh script to run them under stress conditions and race detector to verify their stability.
• [ ] I have added an entry in ./changelog/fragments using the changelog tool.

Disruptive User Impact

Author's Checklist

• [ ]

How to test this PR locally

Related issues

• Requires https://github.com/elastic/elastic-agent-system-metrics/pull/274
• Closes #47604

Use cases

Screenshots

Logs

[github-actions[bot]]

:robot: GitHub comments

Just comment with:

• run docs-build : Re-trigger the docs validation. (use unformatted text in the comment!)

[mergify[bot]]

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

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

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
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.

[github-actions[bot]]

🔍 Preview links for changed docs

• docs/reference/metricbeat/exported-fields-system.md
• docs/reference/metricbeat/metricbeat-metricset-system-process.md

[elasticmachine]

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

[colleenmcginnis]

You'll have to regenerate the docs (make update) to get the check-docs check to pass.