PG-1907 Add columns to track parallel worker activity

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

regression/expected/parallel.out

@@ -0,0 +1,6 @@
+--
+-- Tests for parallel statistics
+--
+SELECT setting::integer < 180000 AS skip_test FROM pg_settings where name = 'server_version_num'  \gset
+\if :skip_test
+\quit

regression/expected/parallel_1.out

@@ -0,0 +1,39 @@
+--
+-- Tests for parallel statistics
+--
+SELECT setting::integer < 180000 AS skip_test FROM pg_settings where name = 'server_version_num'  \gset
+\if :skip_test
+\quit
+\endif
+CREATE EXTENSION pg_stat_monitor;
+SET pgsm.track_utility = FALSE;
+-- encourage use of parallel plans
+SET parallel_setup_cost = 0;
+SET parallel_tuple_cost = 0;
+SET min_parallel_table_scan_size = 0;
+SET max_parallel_workers_per_gather = 2;
+CREATE TABLE pgsm_parallel_tab (a int);
+SELECT pg_stat_monitor_reset() IS NOT NULL AS t;
+ t 
+---
+ t
+(1 row)
+
+SELECT count(*) FROM pgsm_parallel_tab;
+ count 
+-------
+     0
+(1 row)
+
+SELECT query,
+  parallel_workers_to_launch > 0 AS has_workers_to_launch,
+  parallel_workers_launched > 0 AS has_workers_launched
+  FROM pg_stat_monitor
+  WHERE query ~ 'SELECT count'
+  ORDER BY query COLLATE "C";
+                 query                  | has_workers_to_launch | has_workers_launched 
+----------------------------------------+-----------------------+----------------------
+ SELECT count(*) FROM pgsm_parallel_tab | t                     | t
+(1 row)
+
+DROP TABLE pgsm_parallel_tab;