Added prometheus metrics

[neufeldtech]

Implements #365

Added two different collectors for Prometheus metrics that can be enabled independently of each other with CLI flags. These metrics are served from /metrics HTTP api endpoint when at least one of the collectors is enabled.

1. Runtime metrics -runtime-metrics
   These runtime metrics are 'common metrics' that come from these collectors:

• NewGoCollector
• NewBuildInfoCollector
• NewProcessCollector

2. Proxy Metrics -proxy-metrics
   These are toxiproxy-specific metrics that expose the internal state/metrics of toxiproxy. See METRICS.md for description of these metrics.

The current proxy metrics look like this:

# HELP toxiproxy_proxy_received_bytes_total The total number of bytes received on a given proxy link in a given direction
# TYPE toxiproxy_proxy_received_bytes_total counter
toxiproxy_proxy_received_bytes_total{direction="downstream",listener="[::]:8080",proxy="my-proxy",upstream="httpbin.org:80"} 463
toxiproxy_proxy_received_bytes_total{direction="upstream",listener="[::]:8080",proxy="my-proxy",upstream="httpbin.org:80"} 89

# HELP toxiproxy_proxy_sent_bytes_total The total number of bytes sent on a given proxy link in a given direction
# TYPE toxiproxy_proxy_sent_bytes_total counter
toxiproxy_proxy_sent_bytes_total{direction="downstream",listener="[::]:8080",proxy="my-proxy",upstream="httpbin.org:80"} 463
toxiproxy_proxy_sent_bytes_total{direction="upstream",listener="[::]:8080",proxy="my-proxy",upstream="httpbin.org:80"} 89

[miry]

I think by default the metrics should be disabled. We should allow to enable metrics.
It would help to have initialization part for metrics, than could be configured.

Some example that looks like covers my comments: https://developpaper.com/add-prometheus-monitoring-indicators-for-go-applications/

[neufeldtech]

I think by default the metrics should be disabled. We should allow to enable metrics. It would help to have initialization part for metrics, than could be configured.

Some example that looks like covers my comments: https://developpaper.com/add-prometheus-monitoring-indicators-for-go-applications/

It would help to have initialization part for metrics, than could be configured.

I will refactor to make metrics optional (feature flagged) via a command line arg (and likely an environment variable as well, for easy Kubernetes compatibility).

[neufeldtech]

We might need to discuss how to integrate this a little further - I just found another instance of NewProxy() where we don't have access to the server parent struct to get the metrics config data

https://github.com/Shopify/toxiproxy/blob/master/proxy_collection.go#L97-L101

[miry]

Small comments left and linting errors. Overall it looks awesome.