I am using Audacity 2.1.0 on Windows 10. I am having a problem with The Metadata. When I export a song as m4a or mp3 and save it, The metadata editor pops up. I fill in the desired fields and click on “OK”. When I view the folder I saved the song in, I notice that the “Contributing artist” and “Year” columns are not populated. I know that I entered data in “Artist Name” and “Year”. I tried adding “Contributing artist” and populating that field. Still not showing on the file. I know that I am able to edit these using “properties”, but that is double duty. I would like to fill it in once and know that it will be there. 
Is there something I need to do?
I can’t help you directly, but I can tell you why this is “difficult”… Different audio formats use different tagging formats and they don’t all support the exact same fields. There’s even more than one version of [u]ID3[/u] ID3 (used for MP3).
Apparently ID3 can be used for M4A, but I don’t know if that’s the default M4A tagging format. I don’t find “Contributing Artist” on the ID3 Wikipedia page, so maybe it doesn’t support that field.
I use [u]Mp3tag[/u] which supports most formats except WAV.
In what app are you viewing the folder you saved the song in?
For MP3 you should if you are using File Explorer see the data for “Year” if you filled in the “Year” field in Audacity and see the data for “Contributing artists” if you filled in the “Artist” field in Audacity. You will not see “Album artist” populated in Windows apps even if you add that as a custom field in Audacity because Windows apps do not see any Audacity custom fields.
You won’t see either Artist or Year for M4A (or for any other formats written by FFmpeg) because of a bug in FFmpeg. Please see underneath the table here Missing features - Audacity Support.
Gale