pgsql: Count SP-GiST index scans in pg_stat statistics.

Count SP-GiST index scans in pg_stat statistics.

Somehow, spgist overlooked the need to call pgstat_count_index_scan().
Hence, pg_stat_all_indexes.idx_scan and equivalent columns never
became nonzero for an SP-GiST index, although the related per-tuple
counters worked fine.

This fix works a bit differently from other index AMs, in that the
counter increment occurs in spgrescan not spggettuple/spggetbitmap.
It looks like this won't make the user-visible semantics noticeably
different, so I won't go to the trouble of introducing an is-this-
the-first-call flag just to make the counter bumps happen in the
same places.

Per bug #17163 from Christian Quest.  Back-patch to all supported
versions.

Discussion: https://postgr.es/m/17163-b8c5cc88322a5e92@postgresql.org

Branch
------
REL9_6_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/9e959f7ed5d5cfcb0852ee56a4c5113a54c52ee1

Modified Files
--------------
src/backend/access/spgist/spgscan.c | 4 ++++
1 file changed, 4 insertions(+)