Each context operates as an independent virtual device, with its own security policy, interfaces, and administrators. You cannot collect interface statistics on different security contexts through the Admin context.
Potential workarounds include:
Assign key major interfaces you want to report on to multiple contexts (for example, interfaces that overlap the admin context and another security context). This may have limitations at the sub-interface level and may not be scalable. It is also only supported with the firewall in routed mode, as it is not possible to have interfaces bound to multiple contexts in transparent mode.
Monitor each context individually as a discrete device via SNMPv3.