pgstat: split out WAL handling from pgstat_{initialize,report_stat}.
A later commit will move the handling of the different kinds of stats into
separate files. By splitting out WAL handling in this commit that later move
will just move code around without other changes.
Author: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/
20220303021600.hs34ghqcw6zcokdh@alap3.anarazel.de