pgsql: Add pg_stat_get_snapshot_timestamp() to show statistics snapshot

Add pg_stat_get_snapshot_timestamp() to show statistics snapshot timestamp.

Per discussion, this could be useful for purposes such as programmatically
detecting a nonresponding stats collector. We already have the timestamp
anyway, it's just a matter of providing a SQL-accessible function to fetch
it.

Matt Kelly, reviewed by Jim Nasby

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/2fb7a75f37d0beca80f45e15736ec8d50064228a

Modified Files
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doc/src/sgml/monitoring.sgml | 8 ++++++++
src/backend/utils/adt/pgstatfuncs.c | 8 ++++++++
src/include/catalog/catversion.h | 2 +-
src/include/catalog/pg_proc.h | 2 ++
src/test/regress/expected/stats.out | 10 +++++++++-
src/test/regress/sql/stats.sql | 6 +++++-
6 files changed, 33 insertions(+), 3 deletions(-)