Add tracing option (#487)

Adds a middleware to enable tracing with OpenTracing/OpenZipkin.
Enabling tracing will have a large impact on performance so it is
not advisable in production.

middleware/trace/trace.go

@@ -0,0 +1,64 @@
+// Package trace implements OpenTracing-based tracing
+package trace
+
+import (
+	"fmt"
+	"sync"
+
+        "golang.org/x/net/context"
+
+	"github.com/miekg/coredns/middleware"
+        "github.com/miekg/dns"
+        ot "github.com/opentracing/opentracing-go"
+        zipkin "github.com/openzipkin/zipkin-go-opentracing"
+)
+
+// Trace holds the tracer and endpoint info
+type Trace struct {
+	Next middleware.Handler
+	ServiceEndpoint string
+	Endpoint string
+	EndpointType string
+	Tracer ot.Tracer
+	Once sync.Once
+}
+
+// OnStartup sets up the tracer
+func (t *Trace) OnStartup() error {
+	var err error
+	t.Once.Do(func() {
+		switch t.EndpointType {
+		case "zipkin":
+			err = t.setupZipkin()
+		default:
+			err = fmt.Errorf("Unknown endpoint type: %s", t.EndpointType)
+		}
+	})
+	return err
+}
+
+func (t *Trace) setupZipkin() error {
+
+	collector, err := zipkin.NewHTTPCollector(t.Endpoint)
+	if err != nil {
+		return err
+	}
+
+	recorder := zipkin.NewRecorder(collector, false, t.ServiceEndpoint, "coredns")
+	t.Tracer, err = zipkin.NewTracer(recorder, zipkin.ClientServerSameSpan(false))
+	if err != nil {
+		return err
+	}
+	return nil
+}
+
+func (t *Trace) Name() (string) {
+	return "trace"
+}
+
+func (t *Trace) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+	span := t.Tracer.StartSpan("servedns")
+	defer span.Finish()
+	ctx = ot.ContextWithSpan(ctx, span)
+	return middleware.NextOrFailure(t.Name(), t.Next, ctx, w, r)
+}