hydra-queue-runner: add simple "up" exporter

There are probably better ways to achieve this (and will likely need to
be refactored a bit to support further metrics).

flake.nix

@@ -565,6 +565,7 @@
               (if lib.versionAtLeast lib.version "20.03pre"
               then nlohmann_json
               else nlohmann_json.override { multipleHeaders = true; })
+              prometheus-cpp
             ];
 
           checkInputs = [