pgsql: pg_buffercache: Add pg_buffercache_summary()

pg_buffercache: Add pg_buffercache_summary()

Using pg_buffercache_summary() is significantly cheaper than querying
pg_buffercache and summarizing in SQL.

Author: Melih Mutlu <m.melihmutlu@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Aleksander Alekseev <aleksander@timescale.com>
Reviewed-by: Zhang Mingli <zmlpostgres@gmail.com>
Discussion: https://postgr.es/m/CAGPVpCQAXYo54Q%3D8gqBsS%3Du0uk9qhnnq4%2B710BtUhUisX1XGEg%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/2589434ae0fbfe08e46b6a4ffba400140b636074

Modified Files
--------------
contrib/pg_buffercache/Makefile                    |   3 +-
contrib/pg_buffercache/expected/pg_buffercache.out |  33 ++++++
contrib/pg_buffercache/meson.build                 |   1 +
.../pg_buffercache/pg_buffercache--1.3--1.4.sql    |  17 +++
contrib/pg_buffercache/pg_buffercache.control      |   2 +-
contrib/pg_buffercache/pg_buffercache_pages.c      |  67 ++++++++++++
contrib/pg_buffercache/sql/pg_buffercache.sql      |  18 ++++
doc/src/sgml/pgbuffercache.sgml                    | 114 ++++++++++++++++++++-
8 files changed, 250 insertions(+), 5 deletions(-)