prometheus-shelly-exporter: add NixOS module

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

@@ -64,6 +64,7 @@ let
     "rspamd"
     "rtl_433"
     "script"
+    "shelly"
     "snmp"
     "smartctl"
     "smokeping"

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

@@ -0,0 +1,27 @@
+{ config, lib, pkgs, options }:
+
+with lib;
+
+let
+  cfg = config.services.prometheus.exporters.shelly;
+in
+{
+  port = 9784;
+  extraOpts = {
+    metrics-file = mkOption {
+      type = types.path;
+      description = lib.mdDoc ''
+        Path to the JSON file with the metric definitions
+      '';
+    };
+  };
+  serviceOpts = {
+    serviceConfig = {
+      ExecStart = ''
+        ${pkgs.prometheus-shelly-exporter}/bin/shelly_exporter \
+          -metrics-file ${cfg.metrics-file} \
+          -listen-address ${cfg.listenAddress}:${toString cfg.port}
+      '';
+    };
+  };
+}

nixos/tests/prometheus-exporters.nix

@@ -1060,6 +1060,20 @@ let
       '';
     };
 
+    shelly = {
+      exporterConfig = {
+        enable = true;
+        metrics-file = "${pkgs.writeText "test.json" ''{}''}";
+      };
+      exporterTest = ''
+        wait_for_unit("prometheus-shelly-exporter.service")
+        wait_for_open_port(9784)
+        wait_until_succeeds(
+            "curl -sSf 'localhost:9784/metrics'"
+        )
+      '';
+    };
+
     script = {
       exporterConfig = {
         enable = true;