[Patch] New pg_stat_tablespace view

Hi hackers,

I’ve been working on extending the cumulative statistics system to
provide better visibility into tablespace-level workloads, and I'd
like to propose a patch to add a new system view: pg_stat_tablespace.

Currently, PostgreSQL provides statistics per database (e.g.,
pg_stat_database) and per relation (e.g., pg_statio_user_tables).
However, because tablespaces can span multiple databases, it is
difficult for DBAs to analyze storage hotspots across the cluster or
verify if a specific tablespace (such as a high-performance SSD vs a
slow HDD array) is experiencing I/O bottlenecks or excessive temporary
file usage.

The pg_stat_tablespace view bridges this gap by providing an aggregate
view of block I/O and temporary file usage grouped by tablespace,
making it easier to optimize storage architectures.

Thanks,
Shihao