pg_stat_monitor: Performance improvement of pg_stat_monitor.

As we already have quite a lot in pg_stat_monitor. We need to look back and see what we can do to improve the performance of pg_stat_monitor. There are two aspects of performance improvements.

1 - Query time. The pg_stat_monitor should cause minimum query monitoring overhead.
2 - pg_stat_monitor view: There should be no lag when querying the pg_stat_monitor view. 

We need to define acceptance criteria for these items.