Monitoring MegaRAID: Add scraper service

nixos/modules/monitoring/exporters/megacli2prom.nix

+# MegaCli to Prometheus text format exporter
+#
+# Scope: Gets data from MegaRAID compatible storage controllers and mogrifies
+#        to Prometheus text format, saves to a temp file, to later be scraped
+#        by the node exporter.
+# 
+# Usage: Import this to every server with a MegaRAID card that you want to
+#        include in the central monitoring system
+#
+# See https://nixos.org/manual/nixos/stable/#module-services-prometheus-exporters
+
+{ config, lib, ourpkgs }:
+
+let
+  cfg = config.services.private-storage.monitoring.megacli2prom;
+
+in {
+  options.services.private-storage.monitoring.megacli2prom = {
+    enable = lib.mkEnableOption "MegaCli2Prom metrics gathering service";
+    outFile = lib.mkOption {
+      type = lib.types.path;
+      description = "Where to store the temporary file for node exporter to scrape?";
+      default = /run/prometheus-node-exporter/megacli.prom
+    };
+    interval = lib.mkOption {
+      type = lib.types.str;
+      description = ''
+        How often to do it?
+        See https://www.freedesktop.org/software/systemd/man/systemd.time.html#Calendar%20Events
+      '';
+      default = "hourly";
+    };
+  };
+
+  config =
+    lib.mkIf cfg.enable {
+
+      environment.systemPackages = [ ourpkgs.megacli2prom ];
+
+      systemd.services.megacli2prom = {
+        enable = true;
+        description = "MegaCli2Prom metrics gathering service";
+        wantedBy = [ "multi-user.target" ];
+        startAt = cfg.interval;
+        script = "${ourpkgs.megacli2prom}/bin/megacli2prom > ${cfg.outFile}";
+      };
+  };
+}
+