💡 cloudflared Cloudflare Tunnel HTTP Traffic Metrics

**Describe the feature you'd like**

Provide more Prometheus metrics data about the traffic.

**Describe alternatives you've considered**

I run `cloudflared` inside of kubernetes, so I would monitoring the network traffic about pod.

But there is NO way to get detailed metrics like "how much traffic used for ingress and egress with domain `service1.example.com` ".

**Additional context**

Now cloudflared already provides some metrics in proxy/metrics.go, like counter by http response code.

I want more metrics like the counter of proxied traffic data in bytes, for both ingress and egress.

I found that I could append more instrument behavior around `proxy/metrics.go` to accomplish this goal.

Also the well-design of metrics(name, tags) should be considered.

What do you think about it?