pg_stat_statements: Add columns to track parallel worker activity

pg_stat_statements: Add columns to track parallel worker activity

The view pg_stat_statements gains two columns:
- parallel_workers_to_launch, the number of parallel workers planned to
be launched.
- parallel_workers_launched, the number of parallel workers actually
launched.

The ratio of both columns offers hints that parallel workers are lacking
on a per-statement basis, requiring some tuning, in coordination with
"calls", the number of times a query is executed.

As of now, these numbers are tracked within Gather and GatherMerge
nodes.  They could be extended to utilities that make use of parallel
workers (parallel btree and brin, VACUUM).

The module is bumped to 1.12.

Author: Guillaume Lelarge
Discussion: https://postgr.es/m/CAECtzeWtTGOK0UgKXdDGpfTVSa5bd_VbUt6K6xn8P7X+_dZqKw@mail.gmail.com