Monitoring/tahoe metrics collector: Add systemd deps statements

Tryfix the starting of the timer before tahoe is updated.

nixos/modules/monitoring/exporters/tahoe.nix

@@ -43,6 +43,8 @@ in {
         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 ];