[#20997] YSQL, ASH: New wait event for the ASH collector

Summary:
This diff adds a new wait event for the ASH collector.
Jira: DB-9971

Test Plan:
Tested manually

```
yugabyte=# select application_name, wait_event, wait_event_type from pg_stat_activity where application_name = 'yb_ash collector';
 application_name | wait_event | wait_event_type
------------------+------------+-----------------
 yb_ash collector | YbAshMain  | Activity

```

Reviewers: hbhanawat, amitanand

Reviewed By: hbhanawat, amitanand

Subscribers: yql

Differential Revision: https://phorge.dev.yugabyte.com/D36457

src/postgres/src/backend/postmaster/pgstat.c

@@ -3800,6 +3800,9 @@ pgstat_get_wait_activity(WaitEventActivity w)
 		case YB_WAIT_EVENT_QUERY_DIAGNOSTICS_MAIN:
 			event_name = "QueryDiagnosticsMain";
 			break;
+		case WAIT_EVENT_YB_ASH_MAIN:
+			event_name = "YbAshMain";
+			break;
 			/* no default case, so that compiler will warn */
 	}
 

src/postgres/src/backend/utils/misc/yb_ash.c

@@ -625,7 +625,7 @@ YbAshMain(Datum main_arg)
 		int 		rc;
 		/* Wait necessary amount of time */
 		rc = WaitLatch(MyLatch, WL_LATCH_SET | WL_TIMEOUT | WL_POSTMASTER_DEATH,
-					   yb_ash_sampling_interval_ms, PG_WAIT_EXTENSION);
+					   yb_ash_sampling_interval_ms, WAIT_EVENT_YB_ASH_MAIN);
 		ResetLatch(MyLatch);
 
 		/* Bailout if postmaster has died */

src/postgres/src/include/pgstat.h

@@ -836,7 +836,8 @@ typedef enum
 	WAIT_EVENT_WAL_RECEIVER_MAIN,
 	WAIT_EVENT_WAL_SENDER_MAIN,
 	WAIT_EVENT_WAL_WRITER_MAIN,
-	YB_WAIT_EVENT_QUERY_DIAGNOSTICS_MAIN
+	YB_WAIT_EVENT_QUERY_DIAGNOSTICS_MAIN,
+	WAIT_EVENT_YB_ASH_MAIN
 } WaitEventActivity;
 
 /* ----------