It's a major issue for me because of the time investment required to learn the language (functional programming isn't exactly second nature to most programmers)

It's a major issue for me because of the time investment required to learn the language (functional programming isn't exactly second nature to most programmers) and the need to design the monitoring system from scratch. Instead of an out-of-the-box system you're getting more of a framework which allows you to build one. I actually like Clojure and agree that it is readable as well as fun to learn. But I think with something like Prometheus you do more simple configuration than monitoring system design.