Fix systemd dependencies

Thanks go out to @tomprince!

nixos/modules/monitoring/exporters/tahoe.nix

@@ -42,8 +42,6 @@ in {
       systemd.services.tahoe-metrics-collector = {
         enable = true;
         description = "Tahoe metrics gathering service";
-        wantedBy = [ "multi-user.target" ];
-        wants = [ "tahoe.storage.service" ];
         after = [ "tahoe.storage.service" ];
         startAt = cfg.interval;
         path = [ pkgs.curl ];
@@ -56,6 +54,9 @@ in {
           mv "${cfg.outFile}.tmp" "${cfg.outFile}"
         '';
       };
+      systemd.timers.tahoe-metrics-collector = {
+        after = [ "tahoe.storage.service" ];
+      };
   };
 }