prometheus-openvpn-exporter: mark as broken

Prometheus OpenVPN exporter has been broken since OpenVPN 2.5.0 changed
the format of the datetime to ISO8601. After submitting an issue to
upstream, the upstream decided to no longer maintain this exporter.

pkgs/servers/monitoring/prometheus/openvpn-exporter.nix

@@ -16,6 +16,7 @@ buildGoModule rec {
   meta = with lib; {
     inherit (src.meta) homepage;
     description = "Prometheus exporter for OpenVPN";
+    broken = true;
     license = licenses.asl20;
     maintainers = with maintainers; [ fpletz globin ];
   };