pgsql: Add write_to_file to PgStat_KindInfo for pgstats kinds

Add write_to_file to PgStat_KindInfo for pgstats kinds

This new field controls if entries of a stats kind should be written or
not to the on-disk pgstats file when shutting down an instance. This
affects both fixed and variable-numbered kinds.

This is useful for custom statistics by itself, and a patch is under
discussion to add a new builtin stats kind where the write of the stats
is not necessary. All the built-in stats kinds, as well as the two
custom stats kinds in the test module injection_points, set this flag to
"true" for now, so as stats entries are written to the on-disk pgstats
file.

Author: Bertrand Drouvot
Reviewed-by: Nazir Bilal Yavuz
Discussion: https://postgr.es/m/Zz7T47nHwYgeYwOe@ip-10-97-1-34.eu-west-3.compute.internal

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/c06e71d1acaf490660ed4ae5c28b3079628746b0

Modified Files
--------------
src/backend/utils/activity/pgstat.c | 22 +++++++++++++++++++++-
src/include/utils/pgstat_internal.h | 3 +++
.../modules/injection_points/injection_stats.c | 1 +
.../injection_points/injection_stats_fixed.c | 1 +
4 files changed, 26 insertions(+), 1 deletion(-)