Find a replacement for inefficient Prometheus packet counters

[onitake]

This is a follow-up to #32 .

The Prometheus packet counters can cause performance issues when they are called very often.

Find a replacement for them that doesn't impact stream quality.

Ideas:

• Move the counters to metrics/stats.go and update them only periodically
• Consult the github.com/prometheus/client_golang changelog to find out if more efficient version or usage pattern is available