use exometer metrics

This is a new pull request for the Exometer adaptation, with a cleaner commit history (all changes collected into one diff).

Depends on corresponding PRs in riak_core, riak_kv, riak_api, riak_pipe, riak_search, yokozuna.

Highlights of the exometer adaptation:

* Slightly better performance
* Simplified code in riak: less specific knowledge of metric types scattered around in the core, and less need for caching of metrics, since metrics sampling is cheaper.
* Integrated stats reporting support (e.g. via collectd) in exometer
* More flexible function metrics
* Exometer support for custom probes and metrics
* Better metrics query support
* SNMP reporting built into exometer
* Support for disabling existing metrics for further reduced overhead.
* Stat names now have a 'system prefix' (default: 'riak'), to simplify separation of metrics reported from co-located riak_core-based applications. The prefix is turned into a compile-time constant.