Logging and Summary Operations.
get_verbosity(...): Return how much logging output will be produced.
log_every_n(...): Log 'msg % args' at level 'level' once per 'n' times.
log_first_n(...): Log 'msg % args' at level 'level' only first 'n' times.
log_if(...): Log 'msg % args' at level 'level' only if condition is fulfilled.
set_verbosity(...): Sets the threshold for what messages will be logged.