Changes

This PR clarifies the expected value of the used memory attribute on Linux systems, specifying that it should be derived from MemAvailable for a more accurate representation of memory usage.

On Linux, using MemAvailable breaks the assumption that the sum of all system.memory.state values equals system.memory.limit. However, this assumption is only valid because the current used value is derived indirectly from the limit and other memory states (total - free - buffers - cached), which can misrepresent actual memory pressure (with some issues). If the current used state is derivable, I recommend redefining it to reflect a more meaningful value that cannot be derived—specifically one based on MemAvailable—and making system.memory.limit enabled by default.

While the proposed used value can still be calculated from the optional system.memory.limit and system.linux.memory.available metrics, doing so requires awareness of the current used state’s inaccuracy and explicit user configuration. This change aims to promote a more accurate and meaningful metric for memory usage in Linux environments.

Context: shirou/gopsutil#1873
Repository to test the different formulas: https://github.com/rogercoll/linux-memory-metrics

Merge requirement checklist

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