Tracing for gRPC Server (#619)

* Implements tracing in the native gRPC server

* Undo some unnecessary changes

* Properly revert trace/setup.go this time

* Some very very basic tests

* Remove warning for non-Trace middleware

middleware/pkg/trace/trace.go

@@ -0,0 +1,12 @@
+package trace
+
+import (
+	"github.com/coredns/coredns/middleware"
+	ot "github.com/opentracing/opentracing-go"
+)
+
+// Trace holds the tracer and endpoint info
+type Trace interface {
+	middleware.Handler
+	Tracer() ot.Tracer
+}