NOTE: ultimately configuring the device directly caused instability; I had to use the UniFi controller to provision with the option of SNMP enabled to actually get this to work...
BZ.v3.7.58# grep -i snmp /tmp/system.cfg
snmp.community=public
snmp.status=enabled
Background: http://gregsowell.com/?p=3457 https://grafana.com/dashboards/1486
vi /tmp/system.cfg
cfgmtd -f /tmp/system.cfg -w
reboot
https://community.ubnt.com/t5/UniFi-Feature-Requests/publish-SNMP-MIBs/idi-p/828124 https://github.com/pgmillon/observium/tree/master/mibs/ubiquiti
https://share.zabbix.com/network_devices/ubiquiti/unifi-controller