CDAP 544 - MetricsTable API Put to use Long instead of byte array

https://issues.cask.co/browse/CDAP-544
Besides, even if you want to keep this method, it should not need to pass in a `Comparator` (and the signature itself is wrong, it should be `Comparator<? super K>`). You should call `map.comparator()` to use the same Comparator as the original map.