More metrics

hello @phstc, here are some suggestions
which you can incorporate into this gem
if you feel they're good. basically, adds
a bunch of gauge metrics that mirror
[Sidekiq Stats API](https://github.com/mperham/sidekiq/wiki/API).
Re-raising  here causes statsd's `batch` method to never complete.  Batch has to be used inside the individual rescue/ensure blocks.

The specs are written in a way that batch is never tested, so they are passing