refactor(metrics): Move table size metric into lib

[joshua-spacetime]

Query optimization will soon need this metric.
Therefore it must be moved out of core and into lib. Eventually we might want to move all metrics into lib.

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[kulakowski]

Re crate layout, I know @coolreader18 has expressed wanting to move some code around too, between crates. I don't know the best way exactly to set that up, but I think it's about time for us to actually design it out (like, write down that things with X property or needed for Y have to go in Z crate, and the code within it is subject to C constraint because...) and then commit to it within a release cycle.