Refactor metrics

[dapplion]

Motivation

• Our metrics setup is unnecessary verbose, having to declare each metric 3 times in different places
• prom-client native classes do not have sufficient features to allow the gossip validation queue metrics to be separated by labels instead of different metric names. Having a single gossip_validation_queue and then each topic use a different label simplifies a lot the charts in Grafana.

Description

• Create all metrics in functions without return type, to prevent declaring metrics multiple times.
• Create GaugeExtra and HistogramExtra which allow to:
  • Create child classes with pre-defined labels
  • Attach multiple collect() functions after instantiation.
• Rename IBeaconMetrics to IMetrics
• Remove un-used PushGateway code
• Update metric name in Grafana dashboard

[codeclimate]

Code Climate has analyzed commit 0c133c5 and detected 5 issues on this pull request.

Here's the issue category breakdown:

Category	Count
Complexity	2
Duplication	3

View more on Code Climate.

[wemeetagain]

Sad to see the PushGateway code go, but we can always bring it back if/when we need.

Other than that, looks great.