https://docs.oracle.com/en-us/iaas/Content/Compute/Tasks/enablingmonitoring.htm#Enabling_Monitoring_for_Compute_Instances Service gateways or public IP addresses: The compute instance must have either a public IP address or a service gateway to be able to send compute instance metrics to the Monitoring service.
For metric-based autoscaling, monitoring is enabled on the instances in the instance pool, and the Monitoring service is receiving metrics that are emitted by the instance. When you initially create an instance pool using instances that support monitoring, monitoring is enabled by default, regardless of the settings in the pool's instance configuration.