This extension has a direct Metal equivalent in the
`-[MTLDevice sampleTimestamps:gpuTimestamp:]` method. However, that
method returns CPU timestamps in the Mach absolute time domain, which is
*not* that of `CLOCK_MONOTONIC_RAW` but of `CLOCK_UPTIME_RAW`. The
function that corresponds to `CLOCK_MONOTONIC_RAW` is
`mach_continuous_time()`. Therefore, this implementation uses the
`mach_continuous_time()` function for the CPU timestamp. Perhaps we
should lobby the WG for `VK_TIME_DOMAIN_CLOCK_UPTIME_RAW_EXT`.
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