this would depend on how you are doing HA to some extent.
If you are using MS active passive cluster for primary then this cluster would take care of the discovery HA as well as the discovery server is on the primary.
If you are using the UIM HA probe setting up the discovery_sever on your HA hub and leaving it deactivated when not in use and then activating it with the HA during Fail-over should accomplish your needs as well.
With discovery_server 8.5 if you have any LUA scripts setup or manual excludes you would need to move these manually if using UIM with the HA probe.
Hope this helps.