According to the official FreeBSD binaries, their documentation and to what other systems do prometheus comes bundled with consoles and console_libraries. It seems like they are usually located in etc/prometheus/, next to the config file. However they are currently not installed with the package. Maybe it would make sense to add an option, to disable installing them, but I think for most setups it would make sense to have them installed by default.
Currently working on a patch to add these and some other changes to the port :)
A patch adding the default consoles and improving the state of the logging is now waiting for a committer at https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=228185 -David
This is fixed now via https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=228185