[receiver/hostmetrics] Use MemAvailable for Used memory calculation (Linux)

[rogercoll]

Component(s)

receiver/hostmetrics/internal/scraper/memoryscraper

Is your feature request related to a problem? Please describe.

Current memory "used" calculation is based on the old formula used by the free (and procps utils) command on Linux: Used = Total - Free - Buffers - Cached(with SReclaimable)

The issue is that this formula is not accurate because the Cached includes memory that is not freeable. That's why the Linux Kernel 3.14 introduced the MemAvailable state: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=34e431b0ae398fc54ea69ff85ec700722c9da773

Gopsutil (the library used by the receiver to fetch the data) will be switching to the new formula: https://github.com/shirou/gopsutil/pull/1882 (issue

Describe the solution you'd like

Introduce a feature gate that provides a smooth migration for users to the new formula calculation.

Describe alternatives you've considered

No response

Additional context

Semantic Conventions: open-telemetry/semantic-conventions#2534
Ongoing discussion about default memory states: open-telemetry/semantic-conventions#2538

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Pinging code owners:

• receiver/hostmetrics: @dmitryax @braydonk

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