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

Branch
------
master

Details
-------
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(-)