Upgrade to LTS 12.5
Upgrade to LTS 12.5

This requires using the latest git commit for "prometheus-haskell",
which adds support for GHC 8.4, since there is no release for that
library yet.

Doesn't compile yet, since the library also has breaking changes, for
which we need to update our code as well.

channable/icepeak,channable/icepeak,channable/icepeak