proxy-dns no longer sets "metrics" value

This took a bit of digging but it would appear from 2021.1.0 onwards specifying the "metrics" value doesn't really do anything if you're using proxy-dns.

```
root@dns1:~# cloudflared -v
cloudflared version 2020.11.11 (built 2020-11-25-1649 UTC)
root@dns1:~#
root@dns1:~# cat /etc/cloudflared/dns.yml
logfile: /var/log/cloudflared-dns.log
proxy-dns: true
proxy-dns-port: 5354
proxy-dns-address: "::"
metrics: 192.168.200.11:8443
proxy-dns-upstream:
- https://1.1.1.1/dns-query
- https://1.0.0.1/dns-query
root@dns1:~#
root@dns1:~# cat /var/log/cloudflared-dns.log
[2021-02-17T15:20:43.045210782+11:00]: Version 2020.11.11
[2021-02-17T15:20:43.045301771+11:00]: GOOS: linux, GOVersion: go1.15.5, GoArch: arm64
[2021-02-17T15:20:43.045495364+11:00]: Environment variables map[config:/etc/cloudflared/dns.yml logfile:/var/log/cloudflared-dns.log metrics:192.168.200.11:8443 no-autoupdate:true origincert:/etc/cloudflared/cert.pem proxy-dns:true proxy-dns-address::: proxy-dns-port:5354 proxy-dns-upstream:https://1.1.1.1/dns-query, https://1.0.0.1/dns-query]
[2021-02-17T15:20:43.045724062+11:00]: Adding DNS upstream - url: https://1.1.1.1/dns-query
[2021-02-17T15:20:43.045850936+11:00]: Adding DNS upstream - url: https://1.0.0.1/dns-query
[2021-02-17T15:20:43.046724788+11:00]: Starting DNS over HTTPS proxy server on: dns://[::]:5354
**[2021-02-17T15:20:43.048510357+11:00]: Starting metrics server on 192.168.200.11:8443/metrics**

```

Ive checked 2021.1.0;

```
root@dns1:~# cloudflared -v
cloudflared version 2021.1.0 (built 2021-01-13-1549 UTC)
root@dns1:~#

Feb 17 15:50:27 dns1 cloudflared[88317]: #033[90m3:50PM#033[0m #033[32mINF#033[0m Version 2021.1.0
Feb 17 15:50:27 dns1 cloudflared[88317]: #033[90m3:50PM#033[0m #033[32mINF#033[0m GOOS: linux, GOVersion: go1.15.6, GoArch: arm64
Feb 17 15:50:27 dns1 cloudflared[88317]: #033[90m3:50PM#033[0m #033[32mINF#033[0m Environment variables map[config:/etc/cloudflared/dns.yml logfile:/var/log/cloudflared-dns.log metrics:192.168.200.11:8443 no-autoupdate:true origincert:/etc/cloudflared/cert.pem proxy-dns:true proxy-dns-address::: proxy-dns-port:5354 proxy-dns-upstream:https://1.1.1.1/dns-query, https://1.0.0.1/dns-query]
Feb 17 15:50:27 dns1 cloudflared[88317]: #033[90m3:50PM#033[0m #033[32mINF#033[0m Adding DNS upstream #033[36murl=#033[0mhttps://1.1.1.1/dns-query
Feb 17 15:50:27 dns1 cloudflared[88317]: #033[90m3:50PM#033[0m #033[32mINF#033[0m Adding DNS upstream #033[36murl=#033[0mhttps://1.0.0.1/dns-query
Feb 17 15:50:27 dns1 cloudflared[88317]: #033[90m3:50PM#033[0m #033[32mINF#033[0m Starting DNS over HTTPS proxy server #033[36maddress=#033[0mdns://[::]:5354

```

Same result with the most recent as well:

```
root@dns1:~# cloudflared -v
cloudflared version 2021.2.2 (built 2021-02-10-1809 UTC)
root@dns1:~#
{"level":"info","time":"2021-02-17T04:53:42Z","message":"Version 2021.2.2"}
{"level":"info","time":"2021-02-17T04:53:42Z","message":"GOOS: linux, GOVersion: go1.15.7, GoArch: arm64"}
{"level":"info","time":"2021-02-17T04:53:42Z","message":"Settings: map[config:/etc/cloudflared/dns.yml logfile:/var/log/cloudflared-dns.log metrics:192.168.200.11:8443 no-autoupdate:true origincert:/etc/cloudflared/cert.pem proxy-dns:true proxy-dns-address::: proxy-dns-port:5354]"}
{"level":"info","url":"https://1.1.1.1/dns-query","time":"2021-02-17T04:53:42Z","message":"Adding DNS upstream"}
{"level":"info","url":"https://1.0.0.1/dns-query","time":"2021-02-17T04:53:42Z","message":"Adding DNS upstream"}
{"level":"info","address":"dns://[::]:5354","time":"2021-02-17T04:53:42Z","message":"Starting DNS over HTTPS proxy server"}
```

As far as I can see, this issue is only present when using the proxy-dns option, if my config consists of named tunnels and no mentioned of proxy-dns, the metrics server is set correctly.