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

src/backend/replication/logical/conflict.c

@@ -17,8 +17,9 @@
 #include "access/commit_ts.h"
 #include "access/tableam.h"
 #include "executor/executor.h"
+#include "pgstat.h"
 #include "replication/conflict.h"
-#include "replication/logicalrelation.h"
+#include "replication/worker_internal.h"
 #include "storage/lmgr.h"
 #include "utils/lsyscache.h"
 
@@ -114,6 +115,8 @@ ReportApplyConflict(EState *estate, ResultRelInfo *relinfo, int elevel,
 	Assert(!OidIsValid(indexoid) ||
 		   CheckRelationOidLockedByMe(indexoid, RowExclusiveLock, true));
 
+	pgstat_report_subscription_conflict(MySubscription->oid, type);
+
 	ereport(elevel,
 			errcode_apply_conflict(type),
 			errmsg("conflict detected on relation \"%s.%s\": conflict=%s",