simplify BenchmarkAggMetrics*
simplify BenchmarkAggMetrics*

don't test everything all at once
(aggmetric + multiple aggmetrics + different aggregations and schemas,
etc)

let's just test adding to a plain Aggmetric.
we can benchmark other operations (aggregations, etc) separately.
and for a more "complete" picture we should just test an actual running
instance.

raintank/metrictank,raintank/metrictank,raintank/raintank-metric,raintank/metrictank,replay/metrictank,replay/metrictank,raintank/raintank-metric,raintank/metrictank,raintank/raintank-metric,replay/metrictank,replay/metrictank