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]

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

[nekohasekai]

New features need to be designed. Instead of providing an example PR, you should open an issue to discuss what statistics should be provided.

[juvenn]

Ok. Following #87, I noticed pull request welcome is labelled, so I assume a pull-request could better showcase how it look like.

No problem, should I open a new issue to discuss the design? is there a PR example I could look into, or contribution guideline I can follow on?

Thanks for taking a look!

[nekohasekai]

Sorry, I don't have experience with this either, maybe you can refer to the prometheus metrics provided by Clash and make a proposal.