pgsql: Extract logic filling pg_stat_get_io()'s tuplestore into its own

Extract logic filling pg_stat_get_io()'s tuplestore into its own routine

This commit adds pg_stat_io_build_tuples(), a helper routine for
pg_stat_get_io(), that fills its result tuplestore based on the contents
of PgStat_BktypeIO. This will be used in a follow-up commit that uses
the same structures as pg_stat_io for reporting, including the same
object types and contexts, but for a different statistics kind.

Author: Bertrand Drouvot, Michael Paquier
Discussion: https://postgr.es/m/ZtXR+CtkEVVE/LHF(at)ip-10-97-1-34(dot)eu-west-3(dot)compute(dot)internal

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ff7c40d7fd6a218f31fcf6f2c23c544c85934b24

Modified Files
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src/backend/utils/adt/pgstatfuncs.c | 174 ++++++++++++++++++++----------------
1 file changed, 97 insertions(+), 77 deletions(-)