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

https://issues.cask.co/browse/CDAP-544
Looking at the implementation, it is not transforming. It is copying with transformation. Is it the intend? How different is it than the one in guava `Maps`?

```java
  public static <K, V1, V2> NavigableMap<K, V2> transformValues(
      NavigableMap<K, V1> fromMap, Function<? super V1, V2> function) {
```