New function to show index being vacuumed

Hi,

[1] is a ready-for-committer enhancement to pg_stat_progress_vacuum which exposes
the total number of indexes to vacuum and how many indexes have been vacuumed in
the current vacuum cycle.

To even further improve visibility into index vacuuming, it would be beneficial to have a
function called pg_stat_get_vacuum_index(pid) that takes in a pid and returns the
indexrelid of the index being processed.

Currently the only way to get the index being vacuumed by a process
Is through os tools such as pstack.

I had a patch for this as part of [1], but it was decided to handle this in a separate
discussion.

Comments/feedback will be appreciated before sending out a v1 of the patch.

Regards,

Sami Imseih
Amazon Web Services (AWS)

1. https://www.postgresql.org/message-id/flat/5478DFCD-2333-401A-B2F0-0D186AB09228(at)amazon(dot)com