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
Since the the stat_item and stats functions and data types are meant
to be exported, they get an osmo_ prefix.
Sponsored-by: On-Waves ehf
[hfreyther: Prepended the enum values too. This was requested by
Jacob]