Collect statistics about conflicts in logical replication

Hi hackers,
Cc people involved in the related work.

In the original conflict resolution thread[1], we have decided to split the
conflict resolution work into multiple patches to facilitate incremental
progress towards supporting conflict resolution in logical replication, and one
of the work is statistics collection for the conflicts.

Following the discussions in the conflict resolution thread, the collection of
logical replication conflicts is important independently, which can help user
understand conflict stats (e.g., conflict rates) and potentially identify
portions of the application and other parts of the system to optimize. So, I am
starting a new thread for this feature.

The idea is to add columns(insert_exists_count, update_differ_count,
update_exists_count, update_missing_count, delete_differ_count,
delete_missing_count) in view pg_stat_subscription_stats to shows information
about the conflict which occur during the application of logical replication
changes. The conflict types originate from the committed work which is to
report additional information for each conflict in logical replication.

The patch for this feature is attached.

Suggestions and comments are highly appreciated.

[1] https://www.postgresql.org/message-id/CAA4eK1LgPyzPr_Vrvvr4syrde4hyT%3DQQnGjdRUNP-tz3eYa%3DGQ%40mail.gmail.com

Best Regards,
Hou Zhijie