os_mon: Fix use available mem for system_memory_high_watermark alarm

[mmzeeman]

Fixes #8759

• Use available memory (when available) to calculate how much memory a system has used.
• Add available_memory to memsup:get_system_memory_data/1 result on Apple.

Fixes are for Linux` and Apple.

[github-actions]

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[garazdawi]

These changes look good. Do you think the docs need to be updated in order to clarify what is going on?

[mmzeeman]

Yes... that sounds like a good idea. I will add it to the PR.

[mmzeeman]

@garazdawi The currently the documentation goes like this (Source: https://www.erlang.org/doc/apps/os_mon/memsup.html#module-configuration)

The threshold, as percentage of system memory, for how much system memory can be allocated before the corresponding alarm is set. The default is 0.80 (80%).

Which is a probably (intentionally) bit vague. Shall I make it more explicit and say that this is total - available memory on Linux and Apple? Or is that too much detail?

Maybe the release notes should mention the details.

[garazdawi]

I thought I has replied to this... but apparently not.

I think that we should update the docs to be more specific, possibly adding a note saying that the source may change in between releases as better values are exposed by the OS.

[garazdawi]

Thanks!