Add prometheus metrics

[skylenet]

Adding web server to serve /metrics endpoint that can be scraped by prometheus.

Just and example on how it looks like:

$  docker exec -it dns curl localhost:9100/metrics
devp2p_discv4_dns_nodes{domain="all.goerli.nodes.ethflare.xyz"} 242
devp2p_discv4_dns_nodes{domain="all.mainnet.nodes.ethflare.xyz"} 1127
devp2p_discv4_dns_nodes{domain="all.rinkeby.nodes.ethflare.xyz"} 116
devp2p_discv4_dns_nodes{domain="all.ropsten.nodes.ethflare.xyz"} 185
devp2p_discv4_dns_nodes{domain="les.goerli.nodes.ethflare.xyz"} 7
devp2p_discv4_dns_nodes{domain="les.mainnet.nodes.ethflare.xyz"} 62
devp2p_discv4_dns_nodes{domain="les.rinkeby.nodes.ethflare.xyz"} 10
devp2p_discv4_dns_nodes{domain="les.ropsten.nodes.ethflare.xyz"} 5

There's a small breaking change by renaming CRAWL_PUBLISH_METRICS to INFLUXDB_METRICS_ENABLED. This should be updated on any deployment that wants to keep pushing metrics to influx directly.