The problem does not seem to be the Context or the RI or the Mibs....
All of that is configured correctly within the SRX as per the links you provided (thanks), but when then completing the autodiscovery within prtg we get nothing back.
If we complete the walkthrough, as I mentioned, we get the 30,000+ OIDs, but that is really just saying "Hey, SNMP works"....
I don't want to monitor anything attached to the SRX as that is already being monitored, I want to be able to monitor the SRX itself, but we have no default "inet.0" table.... only routing-instances. All the other devices are working really well from an SNMP perspective.... Here is what I configured (I've added a second context to test with same results):
set snmp v3 usm local-engine user ng-sh-engineer authentication-md5 authentication-key <key>
set snmp v3 usm local-engine user ng-sh-engineer privacy-aes128 privacy-key <key>
set snmp v3 vacm security-to-group security-model usm security-name ng-sh-engineer group snmpgroup
set snmp v3 vacm access group snmpgroup context-prefix Customer-VR security-model usm security-level authentication read-view allmibs
set snmp v3 vacm access group snmpgroup context-prefix ninegroup-radius security-model usm security-level authentication read-view allmibs
set snmp engine-id use-default-ip-address
set snmp view allmibs oid .1.3.6.1 include
set snmp view allmibs oid .1 include
set snmp routing-instance-access