identity: provide a log to check the buffers

In order to not rely only on app to display the
message from identity, display the message in the logs
too.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/863>

plugins/elements/gstidentity.c

@@ -707,6 +707,7 @@ gst_identity_update_last_message_for_buffer (GstIdentity * identity,
       GST_BUFFER_FLAGS (buf), flag_str, meta_str ? meta_str : "none", buf);
   g_free (flag_str);
   g_free (meta_str);
+  GST_TRACE_OBJECT (identity, "%s", identity->last_message);
 
   GST_OBJECT_UNLOCK (identity);