Add wait events for checkpoint delay mechanism.

When MyProc->delayChkptFlags is set to temporarily block phase
transitions in a concurrent checkpoint, the checkpointer enters a
sleep-poll loop to wait for the flag to be cleared.  We should show that
as a wait event in the pg_stat_activity view.

Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/CA%2BhUKGL7Whi8iwKbzkbn_1fixH3Yy8aAPz7mfq6Hpj7FeJrKMg%40mail.gmail.com

Author: macdice

src/backend/access/transam/xlog.c

@@ -6737,7 +6737,9 @@ CreateCheckPoint(int flags)
 	{
 		do
 		{
+			pgstat_report_wait_start(WAIT_EVENT_CHECKPOINT_DELAY_START);
 			pg_usleep(10000L);	/* wait for 10 msec */
+			pgstat_report_wait_end();
 		} while (HaveVirtualXIDsDelayingChkpt(vxids, nvxids,
 											  DELAY_CHKPT_START));
 	}
@@ -6750,7 +6752,9 @@ CreateCheckPoint(int flags)
 	{
 		do
 		{
+			pgstat_report_wait_start(WAIT_EVENT_CHECKPOINT_DELAY_COMPLETE);
 			pg_usleep(10000L);	/* wait for 10 msec */
+			pgstat_report_wait_end();
 		} while (HaveVirtualXIDsDelayingChkpt(vxids, nvxids,
 											  DELAY_CHKPT_COMPLETE));
 	}

src/backend/utils/activity/wait_event_names.txt

@@ -97,6 +97,8 @@ BGWORKER_SHUTDOWN	"Waiting for background worker to shut down."
 BGWORKER_STARTUP	"Waiting for background worker to start up."
 BTREE_PAGE	"Waiting for the page number needed to continue a parallel B-tree scan to become available."
 BUFFER_IO	"Waiting for buffer I/O to complete."
+CHECKPOINT_DELAY_COMPLETE	"Waiting for a backend that blocks a checkpoint from completing."
+CHECKPOINT_DELAY_START	"Waiting for a backend that blocks a checkpoint from starting."
 CHECKPOINT_DONE	"Waiting for a checkpoint to complete."
 CHECKPOINT_START	"Waiting for a checkpoint to start."
 EXECUTE_GATHER	"Waiting for activity from a child process while executing a <literal>Gather</literal> plan node."