metrics: fix the maxrss metrics on macOS

[teskje]

On macOS, the ru_maxrss value is for some reason in bytes, not in KiB.

Motivation

• This PR fixes a previously unreported bug.

On macOS, the value of the mz_metrics_libc_ru_maxrss_bytes metric is off by a factor of 1024.

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.

[antiguru]

Thanks! What a mess!

[teskje]

TFTR!