Some metadata are not transfered into the LUA API

Depending on the format of the reading file, some metadata are not transfered into the LUA API, even if their are correctly read ant outup on the GUI. One example is the metadata 'DISCNUMBER' for the MP4 file, but there are others depending of various files formats.

1. Install the lua extension script "Now Playing in Texts" (https://github.com/un-pogaz/Now-Playing-in-Texts)
2. Create/edit a MP4 file with a 'DISCNUMBER' metadata value
3. Read the MP4 file into VLC
4. Start the lua extension "Now Playing in Texts v2"
5. Into the VLC config folder, open the file np_metadata_full.txt
6. The output value '{discnumber}' is empty

VLC 3.0.20 "Vetinari", Windows 10