Hello
You are right, in contrib/pg_buffercache/pg_buffercache--1.2--1.3.sql we have
GRANT EXECUTE ON FUNCTION pg_buffercache_pages() TO pg_monitor;
GRANT SELECT ON pg_buffercache TO pg_monitor;
Not pg_read_all_stats. I'm not sure: we need change the extension or fix the documentation? I think pg_read_all_stats would be more appropriate, but we need bump the extension version.
regards, Sergei