TLDR: Our naming is a mess.
In a nut shell, the phrase "MediaWiki" used in PHP packages or PHP namespaces generally means "MediaWiki-specific", which libraries are not, so we vendor libraries as being "from Wikimedia". On Phabricator on the other hand, "Wikimedia" tends to mean "WMF-specific" which libraries are not, and "MediaWiki-" generally means a component within MediaWiki. And then there is Git-repository permissions which are again based around "mediawiki".
E.g. #mediawiki-jobqueue (core software) vs #wikimedia-jobqueue (wmf infra) vs #mediawiki-libs-thing (reusable library used by core, published as wikimedia/thing),
See the MediaWiki-General description for a list of examples.
I've created this as "MediaWiki-libs-Metrics" which is in line with the others, although maybe in the future we can free up the "Wikimedia-" prefix in Phab from WMF-specific things and call those as "WMF-" instead...