Monitoring the ASA memory usage, when its configured in context mode can apparently not be done though the admin context using snmp.
The workaround is pretty simple. You use the http interface to get the information you want.
Then you will need to import that to your monitoring tool using. (tested and works on PRTG using scripts)
Example to get the memory usage, using http request to the admin context.
https://[admin ip address]/admin/exec//changeto%20context%20system/show%20mem
