EPP should allow configurable metrics collection

[nirrozenbaum]

once we add the notion of Filter and Scorer to EPP, I envisage that orgs that use GIE will have their own filters and scorers in some internal repo.
while GIE is not expected to support dynamic changes of filters/scorers, I believe we do want to allow an easy way to setup different filters/scorers in a fork. That is, I expect the filters/scorers to get registered in the code in a single place (e.g., single file), such that others can change their filters/scorers without drifting from upstream.
With that in mind, the main logic in a scorer (and/or filter) is usually based on some metric(s). so basically if one wants to define his own scorer, he might need to define also additional metric(s).
same as I expect that filters/scorers setup can be easily replaced without drifting from upstream, I think we should allow a very easy, simple and straight forward way to define the set of metrics (that eventually filter/scorer will use) that EPP collects.

[nirrozenbaum]

seems related to this comment #683 (comment)