NETOBSERV-1533: refine metrics for dashboard creation (#281)

* NETOBSERV-1533: refine metrics for dashboard creation

- Use just two shared metrics for eviction counters: on for eviction
  events and one for flows; shared across ringbuf/map implementations
and labelled as such
- Report more errors via metrics

* Fix shared metrics

- Use a single metrics object that holds shared metrics
- Create shared metrics at startup

* Add more metrics:

- accounter and deduper buffer gauges
- use eviction metrics for exporters
- eviction from deduper
- limiter drops counter

* Use a different metric for ringbuf eviction counter

* Add map size gauges