ASA's SNMP setup is similar to IOS but scoped to whichever interface you configure it on, which is easy to get wrong on a multi-context firewall.
Configure the community and host access scoped to the interface the sensor will actually reach the ASA on.
snmp-server host mgmt 10.20.0.50 community linivoRO version 2c snmp-server community linivoRO
Each security context on a multi-context ASA has its own SNMP configuration; enabling it in the admin context doesn't extend to the others.
ASA's MIB reports connection count, interface throughput and failover state for an HA pair; failover-state changes are worth alerting on immediately rather than waiting for the next scheduled poll.