pgsql: Collect statistics about conflicts in logical replication.

Collect statistics about conflicts in logical replication.

This commit adds columns in view pg_stat_subscription_stats to show the
number of times a particular conflict type has occurred during the
application of logical replication changes. The following columns are
added:

confl_insert_exists:
        Number of times a row insertion violated a NOT DEFERRABLE unique
        constraint.
confl_update_origin_differs:
        Number of times an update was performed on a row that was
        previously modified by another origin.
confl_update_exists:
        Number of times that the updated value of a row violates a
        NOT DEFERRABLE unique constraint.
confl_update_missing:
        Number of times that the tuple to be updated is missing.
confl_delete_origin_differs:
        Number of times a delete was performed on a row that was
        previously modified by another origin.
confl_delete_missing:
        Number of times that the tuple to be deleted is missing.

The update_origin_differs and delete_origin_differs conflicts can be
detected only when track_commit_timestamp is enabled.

Author: Hou Zhijie
Reviewed-by: Shveta Malik, Peter Smith, Anit Kapila
Discussion: https://postgr.es/m/OS0PR01MB57160A07BD575773045FC214948F2@OS0PR01MB5716.jpnprd01.prod.outlook.com

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/6c2b5edecc0d6c936e27775c9451d32bb3141c90

Modified Files
--------------
doc/src/sgml/logical-replication.sgml            | 17 +++---
doc/src/sgml/monitoring.sgml                     | 77 +++++++++++++++++++++++-
src/backend/catalog/system_views.sql             |  6 ++
src/backend/replication/logical/conflict.c       |  5 +-
src/backend/utils/activity/pgstat_subscription.c | 17 ++++++
src/backend/utils/adt/pgstatfuncs.c              | 33 +++++++---
src/include/catalog/catversion.h                 |  2 +-
src/include/catalog/pg_proc.dat                  |  6 +-
src/include/pgstat.h                             |  4 ++
src/include/replication/conflict.h               |  8 +++
src/test/regress/expected/rules.out              |  8 ++-
src/test/subscription/t/026_stats.pl             | 61 +++++++++++++------
12 files changed, 204 insertions(+), 40 deletions(-)