Monitoring roles patch

Per the discussion at
https://www.postgresql.org/message-id/CA%2BOCxoyYxO%2BJmzv2Micj4uAaQdAi6nq0w25BPQgLLxsrvTmREw%40mail.gmail.com,
attached is a patch that implements the following:

- Adds a default role called pg_monitor
- Gives members of the pg_monitor role full access to:
pg_ls_logdir() and pg_ls_waldir()
pg_stat_* views and functions
pg_tablespace_size() and pg_database_size()
Contrib modules:
pg_buffercache,
pg_freespacemap,
pgrowlocks,
pg_stat_statements,
pgstattuple and
pg_visibility (but NOT pg_truncate_visibility_map() )
- Adds a default role called pg_read_all_gucs
- Allows members of pg_read_all_gucs to, well, read all GUCs
- Grants pg_read_all_gucs to pg_monitor

Note that updates to contrib modules followed the strategy recently
used in changes to pgstattuple following discussion here, in which the
installation SQL script is left at the prior version, and an update
script is added and default version number bumped to match that of the
upgrade script.

Patch includes doc updates, and is dependent on my pg_ls_logdir() and
pg_ls_waldir() patch
(https://www.postgresql.org/message-id/CA+OCxow-X=D2fWdKy+HP+vQ1LtrgbsYQ=CshzZBqyFT5jOYrFw@mail.gmail.com).

--
Dave Page
Blog: http://pgsnake.blogspot.com
Twitter: @pgsnake

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