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).
@phstc Just looked closer at the operation of `batch` and I believe it will work properly in ensuring the buffer is flushed even though you're re-raising an error.

I'm not sure why I had a problem in my testing as I lost the data now, but it could be because are not doing a full `rescue Exception => e`, and thus you miss out on some types of exceptions.