nixos/tests/prometheus-exporters: Add a test to cover a regression.

nixos/tests/prometheus-exporters.nix

@@ -1123,6 +1123,10 @@ let
     systemd = {
       exporterConfig = {
         enable = true;
+
+        extraFlags = [
+          "--collector.enable-restart-count"
+        ];
       };
       metricProvider = { };
       exporterTest = ''
@@ -1133,6 +1137,11 @@ let
                 'systemd_unit_state{name="basic.target",state="active",type="target"} 1'
             )
         )
+        succeed(
+            "curl -sSf localhost:9558/metrics | grep '{}'".format(
+                'systemd_service_restart_total{state="prometheus-systemd-exporter.service"} 0'
+            )
+        )
       '';
     };