pmrep: rewrite output methods
pmrep: rewrite output methods

Start using the previously added pmconfig support for
fetching only the filtered metric instances (d51f29a),
rearrange code to make it finally almost readable, and
simplify / optimize a bit.

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp