Use gst_element_class_set_metadata when passing dynamic strings
gst_element_class_set_metadata is meant to only be used with static or inlined strings, which isn't the case for the 2 elements here resulting in use-after-free later on.
gst_element_class_set_metadata is meant to only be used with static or inlined strings, which isn't the case for the 2 elements here resulting in use-after-free later on.