clamav_database_age should be a timestamp

[lapo-luchini]

I would suggest changing
clamav_database_age 13580.878680335
to a full timestamp in seconds
clamav_database_age 1677590624770.1213
as it is a value that doesn't change over time and has the same content.
Users can calculate the delta at query time easily enough.

[jocelynthode]

Hey I wanted this change as well as updated images and we've forked the repository here since this one seems completely dead.

https://github.com/liip-elearning/clamav-prometheus-exporter

Feel free to use the image we build on dockerhub at https://hub.docker.com/r/sysadminliip/clamav-prometheus-exporter