I wrote a script to expose stats from unbound to SNMP and built a Cacti template for that. Recently started moving the DNS stats to feed into Telegraf that pushes to an InfluxDB server, then built a dashboard in Grafana. We track DNS RTT for a few queries, number of drops, number of rejects, various record type requests per second, etc. We also have a Nagios plugin that checks each of our DNS cache resolving servers scattered across the network to ensure they can resolve a handful of popular domains.