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

core/dnsserver/server_test.go

@@ -0,0 +1,26 @@
+package dnsserver
+
+import (
+	"testing"
+)
+
+func makeConfig(transport string) *Config {
+	return &Config{Zone: "example.com", Transport: transport, ListenHost: "127.0.0.1", Port: "53"}
+}
+
+func TestNewServer(t *testing.T) {
+	_, err := NewServer("127.0.0.1:53", []*Config{makeConfig("dns")})
+	if err != nil {
+		t.Errorf("Expected no error for NewServer, got %s.", err)
+	}
+
+	_, err = NewServergRPC("127.0.0.1:53", []*Config{makeConfig("grpc")})
+	if err != nil {
+		t.Errorf("Expected no error for NewServergRPC, got %s.", err)
+	}
+
+	_, err = NewServerTLS("127.0.0.1:53", []*Config{makeConfig("tls")})
+	if err != nil {
+		t.Errorf("Expected no error for NewServerTLS, got %s.", err)
+	}
+}