pgsql: Add timestamp of last received message from standby to pg_stat_r

Add timestamp of last received message from standby to pg_stat_replication

The timestamp generated by the standby at message transmission has been
included in the protocol since its introduction for both the status
update message and hot standby feedback message, but it has never
appeared in pg_stat_replication. Seeing this timestamp does not matter
much with a cluster which has a lot of activity, but on a mostly-idle
cluster, this makes monitoring able to react faster than the configured
timeouts.

Author: MyungKyu LIM
Reviewed-by: Michael Paquier, Masahiko Sawada
Discussion: https://postgr.es/m/1657809367.407321.1533027417725.JavaMail.jboss@ep2ml404

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/7fee252f6fbf78ca5e50ee591573d59f98e75d37

Modified Files
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doc/src/sgml/monitoring.sgml | 5 ++
src/backend/catalog/system_views.sql | 3 +-
src/backend/replication/walsender.c | 71 +++++++++++++++++++++++------
src/include/catalog/catversion.h | 2 +-
src/include/catalog/pg_proc.dat | 6 +--
src/include/replication/walsender_private.h | 5 ++
src/test/regress/expected/rules.out | 5 +-
7 files changed, 77 insertions(+), 20 deletions(-)