From http://build.fhir.org/devicemetric-definitions.html#DeviceMetric.parent and  http://build.fhir.org/devicemetric-definitions.html#DeviceMetric.source, the distinction between these fields isn't clear. Todd cooper shared the following disambiguation, but it looks liek the docs all live in the PcD IG; it would be good to migrate this content into the core spec.

Check out the PoCD IG HERE.
 
If you remember the containment model (on THIS PAGE OF THE POCD IG) the containment from Observation => DeviceMetric => Channel => VMD => MDS.
 
Per the PoCD FHIR IG, to navigate:

Observation.device => DeviceMetric

DeviceMetric.source =>  MDS Device  (navigate directly to the top level containment)

DeviceMetric.parent => Channel Device

Channel Device.parent => VMD Device

VMD Device.parent => MDS Device