The question arose on raspbian, but I think that applies to all debian and apt.
In the repository directory I see two versions of the package.
[ ] prometheus_1.5.2+ds-2_armhf.deb 21-Feb-2017 10:08 4.3 M Details
[ ] prometheus_2.7.1+ds-3_armhf.deb 21-Feb-2019 10:08 9.2 M Details
but apt get, search, second versions (2.7.1) I stubbornly refused to show.
apt install prometheus=2.7.1
Reading package lists... Done
Building dependency tree
Reading state information... Ready
E: Version "2.7.1" to "prometheus" was not found
Which generally regulates these things?