enable! method needed for conditionally enabling l2meter output in tests

If I do the recommended:

```ruby
RSpec.configure do |config|
config.around :each do |example|
Metrics.silence &example
end
end
```
I'd also like to test in particular contexts that the Librato metrics I expected are being output. A simple way to do this would be a new method like `disable!` called `enable!` that would allow me to enable L2meter (un-silence it) for just one context in RSpec. It feels like it would be overkill to have to wrap all my other tests in `Metrics.silence` in each context just so that one or two tests can have it enabled.

I suppose another way to test this would be to check that L2Meter is receiving the message I am sending, but I'd rather test this at the logline level, since my tests are already at that level.

Thoughts?