Normalize for direct Prometheus compatibility

[bwhitehead0]

• Support two output formats: JSON, Prometheus
• Support /metrics endpoint for Prometheus

Requirements:

1. Accept argument for format: -format=JSON or -format=Prometheus, case insensitive,
2. Use prometheus libraries below to simplify http server setup etc.

github.com/prometheus/client_golang/prometheus
github.com/prometheus/client_golang/prometheus/promauto
github.com/prometheus/client_golang/prometheus/promhttp