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
3 files changed