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

https://issues.cask.co/browse/CDAP-544
BTW, these two methods in Bytes are not that useful. It should be changed to:

```java
  /**
   * Creates immutable sorted map with one entry with given key and value.
   * @param key key of the entry
   * @param value value of the entry
   * @return instance of {@link NavigableMap}
   */
  public static <T> NavigableMap<byte[], T> immutableSortedMapOf(byte[] key, T value) {
    return ImmutableSortedMap.<byte[], T>orderedBy(Bytes.BYTES_COMPARATOR).put(key, value).build();
  }

  /**
   * Creates immutable sorted map with two entries with given keys and values.
   * @param key1 key of the first entry
   * @param value1 value of the first entry
   * @param key2 key of the second entry
   * @param value2 value of the second entry
   * @return instance of {@link NavigableMap}
   */
  public static <T> NavigableMap<byte[], T> immutableSortedMapOf(byte[] key1, T value1,
                                                                  byte[] key2, T value2) {
    return ImmutableSortedMap.<byte[], T>orderedBy(Bytes.BYTES_COMPARATOR)
      .put(key1, value1)
      .put(key2, value2).build();
  }
```