refactor stat_item: get rid of FIFO and "skipped" error

Intead of attempting to store all distinct values of a reporting period,
just store min, max, last as well as a sum and N of each reporting
period.

This gets rid of error messages like

  DLSTATS ERROR stat_item.c:285 num_bts:oml_connected: 44 stats values skipped

while at the same time more accurately reporting the max value for each
reporting period. (So far stats_item only reports the max value; keep
that part unchanged, as shown in stats_test.c.)

With the other so far unused values (min, sum), we are ready to also
report the minimum value as well as an average value per reporting
period in the future, if/when our stats reporter allows for it.

Store the complete record of the previous reporting period. So far we
only compare the 'max' value, but like this we are ready to also see
changes in min, last and average value between reporting periods.

This patch breaks API by removing:
- struct members osmo_stats_item.stats_next_id, .last_offs and .values[]
- struct osmo_stats_item_value
- osmo_stat_item_get_next()
- osmo_stat_item_discard()
- osmo_stat_item_discard_all()
and by making struct osmo_stats_item opaque.
In libosmocore, we do have a policy of never breaking API. But since the
above should never be accessed by users of the osmo_stats_item API -- or
if they are, would no longer yield useful results, we decided to make an
exception in this case. The alternative would be to introduce a new
osmo_stats_item2 API and maintaining an unused legacy osmo_stats_item
forever, but we decided that the effort is not worth it. There are no
known users of the removed items.

Related: SYS#5542
Change-Id: I137992a5479fc39bbceb6c6c2af9c227bd33b39b

1 files changed, 5 insertions, 1 deletions

diff --git a/TODO-RELEASE b/TODO-RELEASE
index c8966cd3..0f202e33 100644
--- a/TODO-RELEASE
+++ b/TODO-RELEASE
@@ -15,4 +15,8 @@ libosmovty      vty_read_config_filep   New API
 libosmosim	osim_card_{reset,close}	New API
 libosmocore     struct rate_ctr_group, osmo_stat_item_group_desc ABI breakage due to new struct members
 libosmgsm      kdf functions   New API
-libosmocore	osmo_stat_item		ABI + API breakage due to new struct members
+libosmocore	osmo_stat_item		API breakage: remove members stats_next_id, last_offs and values[], no users should exist.
+libosmocore	osmo_stat_item		API breakage: remove functions osmo_stat_item_get_next(), osmo_stat_item_discard(), osmo_stat_item_discard_all(), no users should exist.
+libosmocore	osmo_stat_item_value	API breakage: struct definition removed, because no known users exist / no users should exist.
+libosmocore	osmo_stat_item		ABI breakage: struct osmo_stat_item made opaque.
+libosmocore	osmo_stat_item		No FIFO buffer of values used anymore, the "skipped values" error is no longer possible.