Description: Avoid a spurious dependency on a second protobuf library.

--- a/prometheus/wrap_test.go
+++ b/prometheus/wrap_test.go
@@ -19,7 +19,7 @@
 	"strings"
 	"testing"
 
-	"github.com/gogo/protobuf/proto"
+	"github.com/golang/protobuf/proto"
 
 	dto "github.com/prometheus/client_model/go"
 )