def coalesce( other )
      sync {
        other.stats.each do |name,sampler|
          stats[name].coalesce(sampler)
        end
      }
    end
def coalesce( other )
      sync {
        other.stats.each do |name,sampler|
          stats[name].coalesce(sampler)
        end
      }
    end
Create a new Tracker instance. An optional boolean can be passed in to
 change the "threadsafe" value of the tracker. By default all trackers
 are created to be threadsafe.

 Coalesce the samplers from the _other_ tracker into this one. The
 _other_ tracker is not modified by this method.

 Coalescing the same two trackers multiple times should only be done if
 one of the trackers is reset between calls to this method. Otherwise
 statistics will be counted multiple times.

 Only this tracker is locked when the coalescing is happening. It is
 left to the user to lock the other tracker if that is the desired
 behavior. This is a deliberate choice in order to prevent deadlock
 situations where two threads are contending on the same mutex.