feat: add :51090/metrics endpoint with Prometheus metrics listening on machine IP (#304)

Signed-off-by: Miek Gieben <miek@miek.nl>
Co-authored-by: Pasha Sviderski <me@psviderski.name>

internal/machine/dns/server.go

@@ -15,6 +15,7 @@ import (
 	"time"
 
 	"github.com/miekg/dns"
+	"github.com/psviderski/uncloud/internal/metrics"
 )
 
 const (
@@ -196,6 +197,7 @@ func (s *Server) handleRequest(w dns.ResponseWriter, req *dns.Msg) {
 			log.Error("Failed to forward DNS query.", "err", err)
 			resp = new(dns.Msg).SetRcode(req, dns.RcodeServerFailure)
 		}
+		metrics.DNSQuery.WithLabelValues("false", metrics.Status(err)).Inc()
 
 		s.reply(w, req, resp)
 		return
@@ -232,6 +234,7 @@ func (s *Server) handleRequest(w dns.ResponseWriter, req *dns.Msg) {
 		}
 	}
 	resp.Truncate(maxSize)
+	metrics.DNSQuery.WithLabelValues("true", metrics.Ok).Inc() // NameError is not an error
 
 	s.reply(w, req, resp)
 }