stats/vty: Add selective show stats command

Currently there is only the 'show stats' command which shows all
counter and stat_item values. This can lead to many lines of output
if there are per-subscriber rate counters.

The new command added by this commit allows it to only show groups of
a certain level (class_id), similar to the 'level' configuration
command for stats reporter.

The new command is

  show stats level (global|peer|subscriber)

Sponsored-by: On-Waves ehf
diff --git a/include/osmocom/vty/misc.h b/include/osmocom/vty/misc.h
index f3b46db..97ad4a5 100644
--- a/include/osmocom/vty/misc.h
+++ b/include/osmocom/vty/misc.h
@@ -17,6 +17,8 @@
 			     struct osmo_stat_item_group *statg);
 
 void vty_out_statistics_full(struct vty *vty, const char *prefix);
+void vty_out_statistics_partial(struct vty *vty, const char *prefix,
+	int max_level);
 
 int osmo_vty_write_config_file(const char *filename);
 int osmo_vty_save_config_file(void);