PG-1907 Add columns to track parallel worker activity

These are the same counters as were intorduced in pg_stat_statements in
commit cf54a2c.

pg_stat_monitor.h

@@ -328,6 +328,10 @@ typedef struct Counters
 	Wal_Usage	walusage;
 	int			resp_calls[MAX_RESPONSE_BUCKET];	/* execution time's in
 													 * msec */
+	int64		parallel_workers_to_launch; /* # of parallel workers planned
+											 * to be launched */
+	int64		parallel_workers_launched;	/* # of parallel workers actually
+											 * launched */
 } Counters;
 
 /* Some global structure to get the cpu usage, really don't like the idea of global variable */