RTBKIT-557 added Graphite keys to improve metrics (potentialBiddersPerRe...

Resolved RTBKIT-557, not sure if these are all right. But it seems right to me. Not sure how to test other than in prod? maybe someone can suggest a way to test it. Added the keys: potentialBiddersPerRequest, bidRequestsSentToBiddersPerRequest, bidsPerBidRequest, numRequestWithBid
This won't do exactly what it says. It will count the numbers of bids in 1 bid message from 1 agent. That can happen when an agent bids on multiple impression in the same auction. It probably should expose info.stats->bids instead.