(CDAP-1090) Feature/metrics search

Search for metrics context and available metrics.
https://issues.cask.co/browse/CDAP-1090
This creates 2 new `byte[]` and the next statement creates 3 new `byte[]`. So this method call in total creates 5 `byte[]`, but all you need is one. This seems very expensive, especially you are calling it inside a while loop. Try to reduce the number of new `byte[]`, ideally to zero, because row key length doesn't change, right? You can have the row key `byte[]` passed in and mutates it in this method.