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.
In order to avoid timeouts in case of many clients,
it should be possible to configure which stats should
be collected.
Implements the following sub-collectors:
* topics
* channels
* clients
