tracer: use GST_PTR_FORMAT to log the structure

This way we only expand the structure when we're logging. This allows us to
meassure the pure tracing seperately from the logging.
Also add some comments on further improvements.

gst/gsttracer.c

@@ -203,11 +203,21 @@ gst_tracer_register (GstPlugin * plugin, const gchar * name, GType type)
 void
 gst_tracer_log_trace (GstStructure * s)
 {
-  gchar *data;
-
-  // TODO(ensonic): use a GVariant?
-  data = gst_structure_to_string (s);
-  GST_TRACE ("%s", data);
-  g_free (data);
+  GST_TRACE ("%" GST_PTR_FORMAT, s);
+  /* expands to:
+     gst_debug_log_valist (
+     GST_CAT_DEFAULT, GST_LEVEL_TRACE,
+     file, func, line, object
+     "%" GST_PTR_FORMAT, s);
+     // does it make sense to use the {file, line, func} from the tracer hook?
+     // a)
+     // - we'd need to pass them in the macros to gst_tracer_dispatch()
+     // - and each tracer needs to grab them from the va_list and pass them here
+     // b)
+     // - we create a content in dispatch, pass that to the tracer
+     // - and the tracer will pass that here
+     // ideally we also use *our* ts instead of the one that
+     // gst_debug_log_default() will pick
+   */
   gst_structure_free (s);
 }