pgsql: Add pg_buffercache_usage_counts() to contrib/pg_buffercache.

Add pg_buffercache_usage_counts() to contrib/pg_buffercache.

It was pointed out that pg_buffercache_summary()'s report of
the overall average usage count isn't that useful, and what
would be more helpful in many cases is to report totals for
each possible usage count.  Add a new function to do it like
that.  Since pg_buffercache 1.4 is already new for v16,
we don't need to create a new extension version; we'll just
define this as part of 1.4.

Nathan Bossart

Discussion: https://postgr.es/m/20230130233040.GA2800702@nathanxps13

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f3fa31327ecba75ee0e946abaa56dbf471ba704b

Modified Files
--------------
contrib/pg_buffercache/expected/pg_buffercache.out |  14 +++
.../pg_buffercache/pg_buffercache--1.3--1.4.sql    |  11 +++
contrib/pg_buffercache/pg_buffercache_pages.c      |  43 +++++++++
contrib/pg_buffercache/sql/pg_buffercache.sql      |   4 +
doc/src/sgml/pgbuffercache.sgml                    | 105 ++++++++++++++++++++-
5 files changed, 173 insertions(+), 4 deletions(-)