[MINOR] stats: use 'clear counters all' to clear all values

[MINOR] stats: use 'clear counters all' to clear all values

The most common use of "clear counters" should be to only clear
max values without affecting cumulated values, for instance,
after an incident. So we change "clear counters" to only clear
max values, and add "clear counters all" to clear all counters.