Capture recordHit() metrics for all new auctions (total) and per exchange ...

...as well as ignored per exchange.  Refactor to introduce a helper function in Auction: `const char * exchangeName() const`.

Proposal - follow up with another branch to harmonize the metric naming convention:
* `"bid"` -> `"bids"`
* `"exchange.*.requests"` -> `"exchange.*.auctions"`
* `"exchange.*.ignoredRequests"` -> `"exchange.*.ignoredAuctions"`