static String log10GroupedString(long number) {
    return String.format("%,d", (long) (Math.pow(10d, Math.floor(Math.log10(number)))));
  }
Returns the string representation of the given number after grouping it into log10 buckets.
e.g. 0-9 -> 1, 10-99 -> 10, ..., 100,000-999,999 -> 100,000, etc.
hbase-mapreduce/src/main/java/org/apache/hadoop/hbase/mob/mapreduce/MobRefReporter.java