In order to get the traffic usage info per VM instance, you'll check the 'Monitoring' tab below 'VM instance details' page which is able to give incoming/outgoing network bytes up to last 30 days.
However, if you would like to record network flows sent from and received by VM instances, the best approach would be to use VPC Flow Logs feature.
If enabled for a subnet, VPC flow logs collect data from all VM instances in that subnet.
Also, you'll be able to filter the flow logs by VMs and by applications to know traffic changes.
Furthermore, you can find the destination of the outgoing traffic based on the continent, country, region, city, etc.