feat: pool of connections for dns over tls and https

[qdm12]

• Code complete
• Metrics added
• Full unit test coverage + fuzz test
• Integration test spamming dns on 3 tcp servers, to see how resilient the pool code is done locally, will push later
• update Grafana dashboard
• manually tested to work for dns over tls
• manually tested to work for dns over https (not that necessary though, the *http.Client should already manage this)