Just a question regarding 3rd parties, which are not using Graphite. Do we make Graphite as a dependency now, or we wanna try to make this feature some how not effective if it's (Graphite) not enabled/available?
I'd say we just use mw.track (which is in core) and let each installation decide how they want to handle the data there. In the case of Wikipedia, it seems there is an extra extension that is subscribed to these events and then submits them to our graphite instance.
I would suggest counter.MediaWiki.wikibase.client.databridge.datatype.string, for consistency with some other things tracked in Wikibase. (Note that the MediaWiki part is added automatically when logging from PHP, but in JS we have to add it ourselves.)