Count contrib/bloom index scans in pgstat view.

Maintain the pg_stat_user_indexes.idx_scan pgstat counter during
contrib/Bloom index scans.

Oversight in commit 9ee014fc, which added the Bloom index contrib
module.

Author: Masahiro Ikeda <ikedamsh@oss.nttdata.com>
Reviewed-By: Peter Geoghegan <pg@bowt.ie>
Discussion: https://postgr.es/m/c48839d881388ee401a01807c686004d@oss.nttdata.com
Backpatch: 13- (all supported branches).

contrib/bloom/blscan.c

@@ -121,6 +121,7 @@ blgetbitmap(IndexScanDesc scan, TIDBitmap *tbm)
 	 */
 	bas = GetAccessStrategy(BAS_BULKREAD);
 	npages = RelationGetNumberOfBlocks(scan->indexRelation);
+	pgstat_count_index_scan(scan->indexRelation);
 
 	for (blkno = BLOOM_HEAD_BLKNO; blkno < npages; blkno++)
 	{