metric-api: Add prometheus metrics

[juvenn]

It adds Prometheus metrics api, along with built-in go process metrics, it includes a few more packet metrics

  "experimental": {
    "metrics": {
      "listen": ":8080"
    }
  }

curl localhost:8080/metrics

# HELP inbound_packet_bytes Total bytes of inbound packets
# TYPE inbound_packet_bytes counter
inbound_packet_bytes{inbound="mixed-in",user=""} 1310
# HELP outbound_packet_bytes Total bytes of outbound packets
# TYPE outbound_packet_bytes counter
outbound_packet_bytes{outbound="DIRECT",user=""} 4081
outbound_packet_bytes{outbound="Proxies",user=""} 8704

It should close #87 and serves a starting point for #1564

[juvenn]

Not sure if it is a good name, suggestions are welcome.

[juvenn]

@nekohasekai PTAL, let me know if you've any questions.