This is the minimal program I could think of that could yield the swap
usage on a FreeBSD system. I copied the original from the top utility of
the FreeBSD base and just modified it slightly. If used with the -p
flag, prints a line that is valid for piping to a Prometheus textfile. I
suspect the usefulness of this rather small program would be to avoid
using pstat -T or swapinfo and then having to parse the output with a
script.

Athanasios Douitsis, April 1, 2019