stats: Fix not reported stat item values

Currently the global stat item index is being advanced in
osmo_stat_item_group_handler, so that the second and further groups
in the list will skip new item values.

This commit moves the call to osmo_stat_item_discard_all into
osmo_stats_report to a place where all groups have been processed
already.

Sponsored-by: On-Waves ehf
diff --git a/src/stats.c b/src/stats.c
index d83fd87..16e2971 100644
--- a/src/stats.c
+++ b/src/stats.c
@@ -665,7 +665,6 @@
 static int osmo_stat_item_group_handler(struct osmo_stat_item_group *statg, void *sctx_)
 {
 	osmo_stat_item_for_each_item(statg, osmo_stat_item_handler, sctx_);
-	osmo_stat_item_discard_all(&current_stat_item_index);
 
 	return 0;
 }
@@ -716,10 +715,13 @@
 
 int osmo_stats_report()
 {
+	/* per group actions */
 	osmo_counters_for_each(handle_counter, NULL);
 	rate_ctr_for_each_group(rate_ctr_group_handler, NULL);
 	osmo_stat_item_for_each_group(osmo_stat_item_group_handler, NULL);
 
+	/* global actions */
+	osmo_stat_item_discard_all(&current_stat_item_index);
 	flush_all_reporters();
 
 	return 0;