1) Breaking changes! Updated collection code for proper Prometheus support
- Now metrics names of all subcollectors have prefixes:
nsq_topic_, nsq_channel_, nsq_client_
As a result metrics registration code now passing Prometheus' client validation,
also it fixes scrape warnings
- "type" label in exported metrics removed as obsolete
2) Add reset of gauges before scrape (like in haproxy-exporter)
3) Refactor: subcollectors logic is simplified, multiple collectors support removed
We always use 'stats' collector so it removes unnecessary flexibility and complexity.
