pgsql: pgstat: split out WAL handling from pgstat_{initialize,report_st

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/78f9506b380f0cd3eb2229e2a49040ee00f102bb

Modified Files
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src/backend/postmaster/pgstat.c | 44 ++++++++++++++++++++++++++++-------------
1 file changed, 30 insertions(+), 14 deletions(-)