Fix disk usage data discrepancy on Darwin

[movence]

There is an issue in scraping disk usage on macOS systems. The problem stems from gopsutil's reliance on unix.Statfs, which is not accurately retrieving disk usage data on macOS. While unix.Statfs is generally reliable on many Unix systems, it seems to be incompatible with macOS's specific filesystem (APFS) implementation or reporting mechanisms. This leads to inaccurate disk usage metrics (where all reported mount points have same used space values), impacting macOS customers' observability to their resources.

Why df and not diskutil?
df is preferred due to its universality across unix-like systems including Darwin and consistent/simple output format. While diskutil offers more detailed macOS-specific information, its limited platform support and potential for format changes make it less suitable.

Changes:

• remove darwin from os build target in disk_unix which contains a function to retrieve disk usage
• implement UsageWithContext in disk_darwin using df

Tested on mac1.metal test instance: (screenshot showing all mount points reporting identical values for disk_used metric then showing correct values with this fix)

• disk_used

- disk_used_percent