Metrics::Instrumentable

This adds a module that can be included to provide an `instrument` instance method and class method.

The instance method just delegates to `Metrics.instrument` but the class method let's you do fancy stuff like this:

```ruby
module Some
  class Namespace
    include Metrics::Instrumentable

    def long_method; end

    instrument :long_method
  end
end

Some::Namespace.new.long_method

# In the logs:
# => 'source=app measure.some.namespace.long_method=10s'
```