use exometer metrics

[uwiger]

This is a new pull request for the Exometer adaptation, with a cleaner commit history (all changes collected into one diff).

See riak PR #576

The old riak_pipe PR was #81

[jburwell]

The acceptance task list for this PR is as follows:

Testing

• [ ] manual verification of code
• [ ] eunit (w/ gist of output)
• [ ] EQC (w/ gist of output)
• [ ] riak_test (w/ gist of output)
• [ ] Dialyzer
• [ ] XRef
• [ ] Coverage reports

Documentation

• [ ] internal docs (design docs)
• [ ] external docs (docs.basho.com)
• [ ] man pages

[jburwell]

Superseded by https://github.com/basho/riak_pipe/pull/89