Add wait event for the group commit delay before WAL flush

Author: Rafia Sabih <rafia.pghackers@gmail.com>
Reviewed-by: Sami Imseih <samimseih@gmail.com>
Discussion: https://www.postgresql.org/message-id/CA%2BFpmFf-hWXtrC0Q3Cr_Xo78zuP_M_VC5xgWPOYOkwqOD0T8eg@mail.gmail.com

src/backend/access/transam/xlog.c

@@ -2884,7 +2884,9 @@ XLogFlush(XLogRecPtr record)
 		if (CommitDelay > 0 && enableFsync &&
 			MinimumActiveBackends(CommitSiblings))
 		{
+			pgstat_report_wait_start(WAIT_EVENT_COMMIT_DELAY);
 			pg_usleep(CommitDelay);
+			pgstat_report_wait_end();
 
 			/*
 			 * Re-check how far we can now flush the WAL. It's generally not

src/backend/utils/activity/wait_event_names.txt

@@ -175,6 +175,7 @@ Section: ClassName - WaitEventTimeout
 
 BASE_BACKUP_THROTTLE	"Waiting during base backup when throttling activity."
 CHECKPOINT_WRITE_DELAY	"Waiting between writes while performing a checkpoint."
+COMMIT_DELAY	"Waiting for commit delay before WAL flush."
 PG_SLEEP	"Waiting due to a call to <function>pg_sleep</function> or a sibling function."
 RECOVERY_APPLY_DELAY	"Waiting to apply WAL during recovery because of a delay setting."
 RECOVERY_RETRIEVE_RETRY_INTERVAL	"Waiting during recovery when WAL data is not available from any source (<filename>pg_wal</filename>, archive or stream)."