nix: add missing config option

nix/module.nix

@@ -29,6 +29,7 @@ self: {
     data_dir = cfg.dataDir;
     config_dir = "${cfg.stateDir}/conf";
     cache_dir = cfg.cacheDir;
+    disable_metrics = cfg.disableMetrics;
   });
 in {
   ###### interface
@@ -64,6 +65,12 @@ in {
         example = "10.0.0.5:8080";
       };
 
+      disableMetrics = mkOption {
+        type = bool;
+        default = false;
+        description = "Whether to disable the Prometheus metrics provider for Eris.";
+      };
+
       minDelay = mkOption {
         type = int;
         default = 1000;