refactor(ops): Break out prometheus-fail2ban-exporter module

Break out the configuration for the prometheus fail2ban exporter, which
is a simple python script that exports stats from fail2ban as a
prometheus-scrapable textfile, from Mugwump into a reusable nixos module
in //ops/nixos/modules.

Change-Id: I5451c9c5de6c7bc4431150ae596a9c758bf1b693
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3136
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>

ops/modules/prometheus-fail2ban-exporter.nix

@@ -0,0 +1,52 @@
+{ config, lib, pkgs, depot, ... }:
+
+let
+  cfg = config.services.prometheus-fail2ban-exporter;
+in
+
+{
+  options.services.prometheus-fail2ban-exporter = with lib; {
+    enable = mkEnableOption "Prometheus Fail2ban Exporter";
+
+    interval = mkOption {
+      description = "Systemd calendar expression for how often to run the interval";
+      type = types.string;
+      default = "minutely";
+      example = "hourly";
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    systemd.services."prometheus-fail2ban-exporter" = {
+      wantedBy = [ "multi-user.target" ];
+      after = [ "network.target" "fail2ban.service" ];
+      serviceConfig = {
+        User = "root";
+        Type = "oneshot";
+        ExecStart = pkgs.writeShellScript "prometheus-fail2ban-exporter" ''
+          set -eo pipefail
+          mkdir -p /var/lib/prometheus/node-exporter
+          exec prometheus-fail2ban-exporter
+        '';
+      };
+
+      path = [
+        pkgs.fail2ban
+        depot.third_party.prometheus-fail2ban-exporter
+      ];
+    };
+
+    systemd.timers."prometheus-fail2ban-exporter" = {
+      wantedBy = [ "multi-user.target" ];
+      timerConfig.OnCalendar = cfg.interval;
+    };
+
+    services.prometheus.exporters.node = {
+      enabledCollectors = [ "textfile" ];
+
+      extraFlags = [
+        "--collector.textfile.directory=/var/lib/prometheus/node-exporter"
+      ];
+    };
+  };
+}