PG-186: Add support to monitor query execution plan.

This requires refactoring of code to add this functionality. Along with
that this patch contains regression test cases.

regression/sql/rows.sql

@@ -0,0 +1,19 @@
+CREATE EXTENSION pg_stat_monitor;
+
+CREATE TABLE t1(a int);
+CREATE TABLE t2(b int);
+INSERT INTO t1 VALUES(generate_series(1,1000));
+INSERT INTO t2 VALUES(generate_series(1,5000));
+
+SELECT pg_stat_monitor_reset();
+SELECT * FROM t1;
+SELECT * FROM t2;
+
+SELECT * FROM t1 LIMIT 10;
+SELECt * FROM t2  WHERE b % 2 = 0;
+
+SELECT query, rows FROM pg_stat_monitor ORDER BY query COLLATE "C";
+SELECT pg_stat_monitor_reset();
+
+DROP TABLE t1;
+DROP EXTENSION pg_stat_monitor;