Simplify stats printing

PrintGroupStats and printClusterStats had very similar code
structure. This patch abstracts those two into printStats, making it
a little more generic. Also, printStats has its arguments documented
as requested in previous patch series.

Signed-off-by: Agata Murawska <agatamurawska@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>