Munin support requires additional configuration in the plugin config file:
%%PREFIX%%/etc/munin/plugin-conf.d/plugins.conf

[unbound*]
user root
env.statefile %%PREFIX%%/var/munin/plugin-state/unbound-state
env.unbound_conf %%PREFIX%%/etc/unbound/unbound.conf
env.unbound_control %%PREFIX%%/sbin/unbound-control
env.spoof_warn 1000
env.spoof_crit 100000