Processed metrics metadata 

Just FYI: The coding by convention is rather for the `/core` part of Piwik. For plugins I have always used events see eg recently https://github.com/piwik/piwik/blob/2.9.0-b4/plugins/Installation/Controller.php#L414 and https://github.com/piwik/piwik/blob/master/plugins/Live/Visitor.php#L74 . Eg the last example triggered an event to add visitor details. While I named the file `Visitor.php` in all plugins to easily notice it, it uses an event behind the scenes. Otherwise we'd end in a big mess etc and the `/core` part is basically the platform.

I always worked like this in the last months as a matter of course but should have written/suggested this before.