Problem:
Ubuntu has a different process for installing OpenMetrics and other PMDAs than RPM-based systems. While there are RPMs for the PMDAs, such packages don't exist in DEB format, and are installed manually (which means package_tool can't handle it).
From Google's AI overview (checked on a fresh Ubuntu system):
Once the pcp-zeroconf package is installed (there is a DEB for that):
cd /var/lib/pcp/pmdas/<pmda_name>/
./Install
This should go in setup_pcp in pcp_commands.inc