prometheus_sysctl_exporter(8): filter output using a regex

Add two options, -i and -e, for filtering prometheus metrics.

Each option takes a regular expression as an argument. The provided
regex will be tested against the prometheus metric name.

The -i option includes metrics matching the given regex.
The -e option excludes metrics matching the given regex.

Sponsored by: Modirum MDPay
Sponsored by: Klara Inc.
Reviewed by: 0mp, debdrup
Differential Revision: https://reviews.freebsd.org/D32269