make Trace be a function instead of a method
diff --git a/mirko.go b/mirko.go
index c7778d1..417e45d 100644
--- a/mirko.go
+++ b/mirko.go
@@ -113,7 +113,7 @@
 	return nil
 }
 
-func (m *Mirko) Trace(ctx context.Context, f string, args ...interface{}) {
+func Trace(ctx context.Context, f string, args ...interface{}) {
 	tr, ok := trace.FromContext(ctx)
 	if !ok {
 		fmtd := fmt.Sprintf(f, args...)