I think having documentation in more than one place is an awful experience for newcomers.
Which newcomers are you referring to? Newcomers to the WMF engineering staff or newcomers to research/analytics of Wikimedia projects?
It's OK to not understand the different purposes of our Wikis right away, but I don't think that is a good reason to undermine their purposes. I certainly don't see why wikitech is a desirable hub for this kind of information. From my point of view Wikitech is the *worst* potential hub of information that is not specific to engineering.
What, exactly, is the trouble with having metrics documentation on Meta? How would moving *some of the the documentation* to wikitech help that? (Because you're not going to move research project documentation without even stronger disagreement from the locals.)
-Aaron