Support or clearly indicate as unsupported the drop metrics plugin on AWS VPC CNI

**Is your feature request related to a problem? Please describe.**
AWS VPC CNI can now enforce Network Policy however the e2e test suite shows that in this mode the `drop_count` and `drop_bytes` metrics are not populated even if the plugin is enabled. This may be due to how this metrics is captured by retina vs how this CNI actually functions and drops the filtered packetes. Other parts of the test suite indicate that with `deny-all` network policies enabled packets are actually dropped.

**Describe the solution you'd like**
The metric is populated with zero to pass the e2e tests or actual data is hooked from this CNI to properly populate this metric.

**Describe alternatives you've considered**
Using Calico in IPTables mode in the cluster to drop packets and enable this metric.

**Additional context**
This missing feature was surfaced by WIP e2e test suite in AWS via #667 . Metrics from retina pod during test suite run can be seen [here](https://gist.github.com/whatnick/8a98f712d038957cdadd7c8f4c874e1d)