Some of the snmpcollector devices do not show all metrics in USM although the correct template is active.

I.e. I have discovered a Juniper QFX3500 with snmpcollector but I receive QOS only for internal interfaces, ie: gre, ipip, lsi, mtun, pimd and pime but all the gigabit and 10 gigabit ethernet ports are missing. 

One of the reasons why this may happen is because the device is not certified.

Device certification can be verified here:

UIM SNMP Data Monitoring Device Support 

https://devicesupport.ca.com/?version=2.26  

In this case, Juniper QFX3500, is not supported as per https://devicesupport.ca.com/?version=2.26, however, there is still the option of certifying the  Device by using the SNMP Device Self-Certification: 

The SelfCert portlet is a tool that allows you to add new device and MIB OID support to only your instance of the SNMP Data Monitoring (snmpcollector) probe. Use the SelfCert portlet if existing device metrics are not sufficient, or you have an unsupported SNMP enabled device. Creating a custom vendor certification for a device replaces all the existing metric definitions with a new set. 

https://docops.ca.com/ca-unified-infrastructure-management-probes/ga/en/alphabetical-probe-articles/snmpcollector-snmp-data-monitoring/snmp-device-self-certification