grafana+prometheus: initial setup

machines/gerd/services/monitoring/prometheus.nix

@@ -0,0 +1,27 @@
+{ config, ... }:
+
+let
+  prometheus_user = config.systemd.services.prometheus.serviceConfig.User;
+
+  fullDataDirPath = "/var/lib/${config.services.prometheus.stateDir}";
+
+  filesetPath = config.mine.zfsMounts."rpool/safe/svcs/prometheus";
+in {
+  services.prometheus = {
+    enable = true;
+    globalConfig.scrape_interval = "10s";
+    listenAddress = "localhost";
+
+    # default is 15 days, we just set it to 14 to be explicit
+    retentionTime = "14d";
+  };
+
+  fileSystems."${filesetPath}".neededForBoot = true;
+  environment.persistence."${filesetPath}".directories = [
+    fullDataDirPath
+  ];
+
+  systemd.tmpfiles.rules = [
+    "Z ${fullDataDirPath} 0770 ${prometheus_user} ${prometheus_user} -"
+  ];
+}