This commit moves a part of the gnocchi collector to the base collector,
so that all collectors can benefit from multirating per metric.
It solves the issue with Prometheus having one "*_info" metric containing
the scope and multiple other metric containing the values we want to rate.
Also splitting prometheus' fetch_all, to gain in readability and allow
us to render the promql query from tests.
Change-Id: I4f0892a80f37ce052f573e7622534aa728e7cff1
afa6f98a01