There's a ticket for adding matrix numbers to Recordings (STYLE-754), but, as is also noted on this ticket, this is something different from the matrix on CDs and vinyls.

A number of Releases already have matrix information in their annotation: https://musicbrainz.org/search?query=type%3ARelease+text%3AMatrix&type=annotation&limit=25&method=advanced
And runout: https://musicbrainz.org/search?query=type%3ARelease+text%3ARunout&type=annotation&limit=25&method=advanced

I would suggest adding Runout as a separate attribute, but we could also go the Discogs route of simple specifying "Matrix / Runout".