Add track helper

proof of concept:

```js
import Ember from 'ember';

const {
assign,
inject: { service }
} = Ember;

export default Ember.Helper.extend({
metrics: service(),

compute([ event ], params = {}) {
return () => this.get('metrics').trackEvent(assign({ event }, params));
}
});
```

Usage:

```hbs

Stuff

```

cc @offirgolan

The issue provides a JavaScript Ember helper proof of concept and a Handlebars usage example. Start by checking how this repository exposes helpers and the metrics service, then verify that the shown track invocation handles the event and parameters; done means the demonstrated usage works.

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