Add URL Metric viewer on the frontend

[westonruter]

There shouldn't have to be a need to visit the Edit Post screen to view the URL Metrics for a given page. Why not have the URL Metrics link in the admin bar open up a dialog that has the information presented there?

[westonruter]

There are two key benefits here:

1. We'll have access to the current ETag.
2. We could possibly avoid having to re-construct the URL_Metric_Group_Collection if the OD plugin makes it publicly available (e.g. via a global).