Metric names: It's vouchers (plural)

nixos/modules/monitoring/server/grafana-dashboards/payments.json

@@ -208,7 +208,7 @@
         },
         {
           "exemplar": true,
-          "expr": "payment_redemption_voucher_redeemed",
+          "expr": "payment_redemption_vouchers_redeemed",
           "format": "time_series",
           "hide": false,
           "interval": "",