Fixing sample_checking so that it calls new APIs for parsing Telescope data
Changing the semantics of TelescopeResultReader so that now _read_rows is private and callers get the rows through the __iter__ function. Also got rid of the get_metadata function in MergedTelescopeResultReader because it was confusing and unncessesary.

Updated TelescopeResultMerger to group results according to group key, then metric name.