[PATCH] Support for pg_stat_archiver view

Hello,

please find attached the patch that adds basic support for the
pg_stat_archiver system view, which allows users that have continuous
archiving procedures in place to keep track of some important metrics
and information.

Currently, pg_stat_archiver displays:

* archived_wals: number of successfully archived WAL files since start
(or the last reset)
* last_archived_wal: last successfully archived WAL file
* last_archived_wal_time: timestamp of the latest successful WAL archival
* stats_reset: time of last stats reset

This is an example of output:

postgres=# select * from pg_stat_archiver ;
-[ RECORD 1 ]----------+------------------------------
archived_wals | 1
last_archived_wal | 000000010000000000000001
last_archived_wal_time | 2014-01-04 01:01:08.858648+01
stats_reset | 2014-01-04 00:59:25.895034+01

Similarly to pg_stat_bgwriter, it is possible to reset statistics just
for this context, calling the pg_stat_reset_shared('archiver') function.

The patch is here for discussion and has been prepared against HEAD.
It includes also changes in the documentation and the rules.out test.

I plan to add further information to the pg_stat_archiver view,
including the number of failed attempts of archival and the WAL and
timestamp of the latest failure. However, before proceeding, I'd like to
get some feedback on this small patch as well as advice on possible
regression tests to be added.

Thank you.

Cheers,
Gabriele

--
Gabriele Bartolini - 2ndQuadrant Italia
PostgreSQL Training, Services and Support
gabriele(dot)bartolini(at)2ndQuadrant(dot)it | www.2ndQuadrant.it