If I add an Elastic Network Interface to an EC2 instance, does the traffic flowing over this new interface showing the default CloudWatch EC2 network utilization metrics, or does this only capture traffic from the default network interface (eth0)?
These metrics are across all network interfaces for the host, this is explained for each metric in the List the available CloudWatch metrics for your instances page.
For example the NetworkIn
metric states the following:
The number of bytes received on all network interfaces by the instance.