CISCO-FIREWALL-MIB gives the "show conn" information for current used and most used data.
All the supported MIBs for Cisco ASA can be found at:
ftp://ftp.cisco.com/pub/mibs/supportlists/asa/asa-supportlist.html
This information might also help:
https://supportforums.cisco.com/document/7336/snmp-mibs-and-traps-asa-additional-information#SNMP_MIB_Support
ASA 8.3 and Later: Monitor and Troubleshoot Performance Issues:
http://www.cisco.com/c/en/us/support/docs/security/asa-5500-x-series-next-generation-firewalls/113185-asaperformance.html