Refactor

This does a couple of things.

* Refactor to remove ActiveSupport dependency.
* Adds support for multi-metrics.

Now we can do cool things like this:

```ruby
Metrics.group 'sidekiq' do
  instrument 'queues.process', 100, units: 'jobs'
  instrument 'workers.busy', 10, units: 'workers'

  instrument 'queue.time', source: 'worker.1' do
    sleep 1
  end
end

# => 'source=app measure.sidekiq.queues.processed=100jobs measure.sidekiq.workers.busy=10workers'
# => 'source=app.worker.1 measure.sidekiq.queue.time=1000.00ms'
```

Might need to think about splitting the lines every 5 measurements or so.