Feature/metrics docs

Reopening this PR, against release with addressed comments.
https://github.com/caskdata/cdap/pull/709
every part is options in that aspect, i mean, if flowlet is not specified it will be aggregate by flow, if program type is not specified , it will be aggregated at app level, so i am not going to repeat that here, however i have added that description in comments section, where we describe about different ways of querying.