We have rate_ctr for counters which is somewhat documented in "Common variables" subsection of "Osmocom Control Interface" chapter is osmo-gsm-manuals. The stat_item.h is unfortunately not documented at all. It's very similar in use but unlike rate_ctr it store raw value of some metric. It can be accessed the same way as rate_ctr (see
sh asciidoc) vty command and should be similarly documented.