pgsql: Add system view pg_stat_kind_info

Add system view pg_stat_kind_info

This commit adds support for pg_stat_kind_info, that exposes at SQL
level data about the statistics kinds registered into a backend:
- Meta-data of a stats kind (built-in or custom, some properties).
- Number of entries, if tracking is enabled.

We have discussed the possibility of more fields (like shared memory
size for a single entry); this adds the minimum agreed on.

This is in spirit similar to pg_get_loaded_modules() for custom stats
kinds, this view providing detailed information about the stats kinds
when registered through shared_preload_libraries.

Bump catalog version.

Author: Tristan Partin <tristan(at)partin(dot)io>
Reviewed-by: Bertrand Drouvot <bertranddrouvot(dot)pg(at)gmail(dot)com>
Reviewed-by: Sami Imseih <samimseih(at)gmail(dot)com>
Reviewed-by: Michael Paquier <michael(at)paquier(dot)xyz>
Discussion: https://postgr.es/m/DI6OFGHJ1B69.25YVDEP3BABRH@partin.io

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/3b066de6c0a1dadbd8bed107e55cae659af0598f

Modified Files
--------------
doc/src/sgml/monitoring.sgml | 128 +++++++++++++++++++++
src/backend/catalog/system_views.sql | 11 ++
src/backend/utils/activity/Makefile | 1 +
src/backend/utils/activity/meson.build | 1 +
src/backend/utils/activity/pgstat_kind.c | 72 ++++++++++++
src/include/catalog/catversion.h | 2 +-
src/include/catalog/pg_proc.dat | 8 ++
.../test_custom_stats/t/001_custom_stats.pl | 14 ++-
src/test/regress/expected/rules.out | 8 ++
src/test/regress/expected/stats.out | 23 ++++
src/test/regress/sql/stats.sql | 7 ++
11 files changed, 273 insertions(+), 2 deletions(-)