Interface prototype to collect metadata

Interface prototype to collect integrated service metadata
I made sure that even a failing instance returns metadata (i.e an empty dictionary). 
![screen shot 2014-12-16 at 11 57 21 am](https://cloud.githubusercontent.com/assets/5708089/5457734/d820cb7e-851a-11e4-919d-c55873af324e.png).

The benefit of a list to store metadata, is that it allows to follow the same 'global' approach to collect metadata like events or agent checks. Each service check triggers its instance checks , which, in turn, submits their data/metadata.  Once the check execution is completed, data/metadata are available for the whole service check.