core: remove HostAddresses() (#1728)

* core: remove HostAddresses()

config.HostAddresses() is a weird function that gathers
some data from the server and returns a string.

It is *only* used the trace plugin, to figure out what
server starts the trace.

Looks to be better to fit in the with metrics.WithServer label
on the trace itself to show which server handled the trace.

Remove HostAddresses() and cleanup trace a small bit.:w

* lint

plugin/trace/trace.go

@@ -9,6 +9,7 @@ import (
 	"sync/atomic"
 
 	"github.com/coredns/coredns/plugin"
+	"github.com/coredns/coredns/plugin/metrics"
 	// Plugin the trace package.
 	_ "github.com/coredns/coredns/plugin/pkg/trace"
 
@@ -20,10 +21,10 @@ import (
 
 type trace struct {
 	Next            plugin.Handler
-	ServiceEndpoint string
 	Endpoint        string
 	EndpointType    string
 	tracer          ot.Tracer
+	serviceEndpoint string
 	serviceName     string
 	clientServer    bool
 	every           uint64
@@ -58,7 +59,7 @@ func (t *trace) setupZipkin() error {
 		return err
 	}
 
-	recorder := zipkin.NewRecorder(collector, false, t.ServiceEndpoint, t.serviceName)
+	recorder := zipkin.NewRecorder(collector, false, t.serviceEndpoint, t.serviceName)
 	t.tracer, err = zipkin.NewTracer(recorder, zipkin.ClientServerSameSpan(t.clientServer))
 
 	return err
@@ -81,9 +82,7 @@ func (t *trace) setupDatadog() error {
 }
 
 // Name implements the Handler interface.
-func (t *trace) Name() string {
-	return "trace"
-}
+func (t *trace) Name() string { return "trace" }
 
 // ServeDNS implements the plugin.Handle interface.
 func (t *trace) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
@@ -96,7 +95,7 @@ func (t *trace) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg)
 		}
 	}
 	if span := ot.SpanFromContext(ctx); span == nil && trace {
-		span := t.Tracer().StartSpan("servedns")
+		span := t.Tracer().StartSpan("servedns:" + metrics.WithServer(ctx))
 		defer span.Finish()
 		ctx = ot.ContextWithSpan(ctx, span)
 	}