These errors can be avoided by triggering scaling actions on which ELB metric?

Explanations:

TheHealthyHostCountmetric indicates the number of healthy hosts that can handle requests. While it is important to know the health of your instances, it does not directly help in triggering scaling actions for managing incoming traffic during high load conditions.

BackendConnectionErrorsmeasures the number of connection errors between the load balancer and the backend instances. Although this can help diagnose problems, it does not directly relate to the scaling action needed to handle traffic overflow.

SurgeQueueLengthmeasures the number of requests waiting in the queue for processing. If the surge queue length increases, it means the load balancer is overwhelmed with requests, indicating the need for scaling actions to avoid 5xx errors.

UnHealthyHostCountshows the number of unhealthy instances. While it helps in monitoring instance health, it does not provide insight into whether the system can handle a surge of incoming requests, which is necessary for scaling.