Support Collector middleware using custom metric names (#68)

This commit is cherry-picked from: https://github.com/prometheus/client_ruby/commit/c35e7d88718864711e06127f5868facd882a4dad

And removes the summary metrics as per this commit: https://github.com/prometheus/client_ruby/commit/226451c398c00507acd27cca346b658ef7ad5636#diff-51beee571b6b074e81f2245ca237e446

Original commit message: It can sometimes be handy to expose the HTTP server metrics under a different prefix, either for naming consistency (so all your app metrics have a common prefix), or to prevent conflicts (it is possible, and occasionally useful, to run more than one Rack-based webserver in a single process, although it can be a bit of a wrestle).