Monitor the data transfer bytes in lightsail

I'm using Lightsail instances and I want to avoid getting data transfer costs by turning off the instances before the free data transfer quota is used up. So I want to create a Cloudwatch alarm on the total bytes transferred in a given region. I have created billing alerts but it is usually too late by the time I find out that the data transfer in a region has passed the threshold and even if I turn off the instance the same day, I get charged for tens of dollars. Is there a metric I can use for this?