services/prometheus/exporters: add mysqld

Co-authored-by: Sandro <sandro.jaeckel@gmail.com>

nixos/doc/manual/release-notes/rl-2311.section.md

@@ -18,6 +18,8 @@
 
 - [Anuko Time Tracker](https://github.com/anuko/timetracker), a simple, easy to use, open source time tracking system. Available as [services.anuko-time-tracker](#opt-services.anuko-time-tracker.enable).
 
+- [Prometheus MySQL exporter](https://github.com/prometheus/mysqld_exporter), a MySQL server exporter for Prometheus. Available as [services.prometheus.exporters.mysqld](#opt-services.prometheus.exporters.mysqld.enable).
+
 - [sitespeed-io](https://sitespeed.io), a tool that can generate metrics (timings, diagnostics) for websites. Available as [services.sitespeed-io](#opt-services.sitespeed-io.enable).
 
 - [Apache Guacamole](https://guacamole.apache.org/), a cross-platform, clientless remote desktop gateway. Available as [services.guacamole-server](#opt-services.guacamole-server.enable) and [services.guacamole-client](#opt-services.guacamole-client.enable) services.

nixos/modules/services/monitoring/prometheus/exporters.nix

@@ -49,6 +49,7 @@ let
     "mikrotik"
     "minio"
     "modemmanager"
+    "mysqld"
     "nextcloud"
     "nginx"
     "nginxlog"
@@ -295,6 +296,12 @@ in
         Please specify either 'services.prometheus.exporters.mail.configuration'
           or 'services.prometheus.exporters.mail.configFile'.
       '';
+    } {
+      assertion = cfg.mysqld.runAsLocalSuperUser -> config.services.mysql.enable;
+      message = ''
+        The exporter is configured to run as 'services.mysql.user', but
+          'services.mysql.enable' is set to false.
+      '';
     } {
       assertion = cfg.sql.enable -> (
         (cfg.sql.configFile == null) != (cfg.sql.configuration == null)

nixos/modules/services/monitoring/prometheus/exporters/mysqld.nix

@@ -0,0 +1,60 @@
+{ config, lib, pkgs, options }:
+let
+  cfg = config.services.prometheus.exporters.mysqld;
+  inherit (lib) types mkOption mdDoc mkIf mkForce cli concatStringsSep optionalString escapeShellArgs;
+in {
+  port = 9104;
+  extraOpts = {
+    telemetryPath = mkOption {
+      type = types.str;
+      default = "/metrics";
+      description = mdDoc ''
+        Path under which to expose metrics.
+      '';
+    };
+
+    runAsLocalSuperUser = mkOption {
+      type = types.bool;
+      default = false;
+      description = mdDoc ''
+        Whether to run the exporter as {option}`services.mysql.user`.
+      '';
+    };
+
+    configFile = mkOption {
+      type = types.path;
+      example = "/var/lib/prometheus-mysqld-exporter.cnf";
+      description = mdDoc ''
+        Path to the services config file.
+
+        See <https://github.com/prometheus/mysqld_exporter#running> for more information about
+        the available options.
+
+        ::: {.warn}
+        Please do not store this file in the nix store if you choose to include any credentials here,
+        as it would be world-readable.
+        :::
+      '';
+    };
+  };
+
+  serviceOpts = {
+    serviceConfig = {
+      DynamicUser = !cfg.runAsLocalSuperUser;
+      User = mkIf cfg.runAsLocalSuperUser (mkForce config.services.mysql.user);
+      LoadCredential = mkIf (cfg.configFile != null) (mkForce ("config:" + cfg.configFile));
+      ExecStart = concatStringsSep " " [
+        "${pkgs.prometheus-mysqld-exporter}/bin/mysqld_exporter"
+        "--web.listen-address=${cfg.listenAddress}:${toString cfg.port}"
+        "--web.telemetry-path=${cfg.telemetryPath}"
+        (optionalString (cfg.configFile != null) ''--config.my-cnf=''${CREDENTIALS_DIRECTORY}/config'')
+        (escapeShellArgs cfg.extraFlags)
+      ];
+      RestrictAddressFamilies = [
+        # The exporter can be configured to talk to a local mysql server via a unix socket.
+        "AF_UNIX"
+      ];
+    };
+  };
+}
+