pgsql: Add callback for backend initialization in pgstats

Add callback for backend initialization in pgstats

pgstat_initialize() is currently used by the WAL stats as a code path to
take some custom actions when a backend starts. A callback is added to
generalize the concept so as all stats kinds can do the same, for
builtin and custom kinds, if set.

Reviewed-by: Bertrand Drouvot, Kyotaro Horiguchi
Discussion: /messages/by-id/ZtZr1K4PLdeWclXY@paquier.xyz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/1b373aed20e61e4a3033e1e396e4ba7c2a96bc20

Modified Files
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src/backend/utils/activity/pgstat.c | 14 ++++++++++++--
src/backend/utils/activity/pgstat_wal.c | 2 +-
src/include/utils/pgstat_internal.h | 8 +++++++-
3 files changed, 20 insertions(+), 4 deletions(-)