Fix packet counters and track fragmented packets as parsing issues

[fako1024]

It turns out that the current way of counting processed packets isn't perfectly in line with what a user would expect (probably a side effect of the recent performance enhancements in #284 . Unfortunately the E2E tests for Prometheus counters were silently broken so we didn't see that. 😓

Since this will touch the metrics anyway: The idea for the additional metric would be to add a new packet parsing "error" / issue (similar to the existing one for truncated packets) to help spot issues simply by looking at the interface processing stats.

DoD

• Fix E2E tests for Prometheus counters (and add protection against silent breakage in the future)
• Extend metrics for packet parsing issues to track packet fragments