SRX uses SPUs as the base for forwarding.
Need the CPU Utilization value seen in "show security monitoring fpc X" in OID
For 22.4R3, the information about the OID can be found here:
https://apps.juniper.net/mib-explorer/navigate?software=Junos%20OS&release=22.4R3&name=jnxJsSPUMonitoringCPUUsage&oid=1.3.6.1.4.1.2636.3.39.1.12.1.1.1.4
The name is: jnxJsSPUMonitoringCPUUsage
The OID is: 1.3.6.1.4.1.2636.3.39.1.12.1.1.1.4
If you are in a chassis cluster, you may do a walk for that OID first, to see what extension is which node.