Both concepts are related but separate. I'd argue that monitoring is the act of alerting or taking action when some metric is out of bounds with normal values. Those values are based on absolute thresholds, or some changing thresholds based on historical values. Observability is the ability to see what's happening across and between multiple systems so that any issues raised by monitoring can be diagnosed quickly. "I know theres a problem vs I know which part of the system its happening in"