[mac] expose the memory pressure percentage value

[atoulme]

This adds supports on Darwin system to expose the memory pressure value of the system.

This value is a percentage indicating the memory pressure of the system as a whole, computed by the kernel.

This is based off prior art:
exelban/stats#1453
https://github.com/apple-oss-distributions/xnu/blob/f6217f891ac0bb64f3d375211650a4c1ff8ca1ea/osfmk/arm/commpage/commpage.c#L338
https://github.com/apple-oss-distributions/xnu/blob/f6217f891ac0bb64f3d375211650a4c1ff8ca1ea/osfmk/vm/vm_pageout.c#L1878

[anelson-splunk]

Looks great

[a25salas]

Excellent!

[shirou]

Thank you for taking the time to implement this feature. I really appreciate your effort and contribution.

However, gopsutil aims to be a multi-platform library. We generally do not accept features that are specific to a single OS. At a minimum, new features need to be supported on Windows and Linux as well. And keeping strict alignment with Python's psutil API. Since this feature is macOS-specific and not part of the original psutil, adding it would deviate from our project goals.

Regrettably, I have to reject this PR. I hope you understand our position on maintaining a consistent scope for the library. Thanks again.

[atoulme]

Not a problem. Great to get validation that this might be useful on Mac OS. We will directly implement this feature in the hostmetricsreceiver of the OpenTelemetry Collector for Mac OS X support only.