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

https://issues.cask.co/browse/CDAP-544
If copying of the value map is not needed, you can it in guava:

```java
static final Function<byte[], Long> BYTES_TO_LONG = new Function<byte[], Long>() {
  @Override
  public Long apply(byte[] input) {
    return Bytes.toLong(input);
  }
};
....
NavigableMap<byte[], NavigableMap<byte[], byte[]>> source = Maps.newTreeMap(Bytes.BYTES_COMPARATOR);
NavigableMap<byte[], NavigableMap<byte[], Long>> transformed = Maps.transformValues(source,
  new Function<NavigableMap<byte[], byte[]>, NavigableMap<byte[], Long>>() {
  @Override
  public NavigableMap<byte[], Long> apply(NavigableMap<byte[], byte[]> input) {
    // Do something
    return Maps.transformValues(input, BYTES_TO_LONG);
  }
});
```