Allow consumer to disable tracking by default

Ember Metrics allows to globally enabled and disable tracking through `enabled` property of the metrics service. This is helpful to comply with European privacy rules. It allows a site to disable tracking until the user explicitly confirmed to tracking. Sadly the value of that property is hardcoded as `true`. This forces consumers that want to disable tracking by default into two hacks:

1. Implement a custom metrics service in application namespace, which extends from the one provided by this addon and overrides `enabled` property.
2. Set `enabled` to `false` early in application boot e.g. using an instance initializer or `beforeModel` hook of application route.

It would be better if ember metrics allows to control the default value of that property through a configuration option.