Allow overwriting the empty string labels/types in the Resource. (#231)

Author: Bogdan Drutu

resource/Resource.md

@@ -61,8 +61,8 @@ vendors, MUST be implemented outside of the core libraries in third party or
 ### Merging
 As different mechanisms are run to gain information about a resource, their information
 has to be merged into a single resulting resource.
-Already set labels or type fields MUST NOT be overwritten. Label key namespaceing SHOULD
-be used to prevent collisions across different resource detection steps.
+Already set labels or type fields MUST NOT be overwritten unless they are empty string. Label key
+namespaceing SHOULD be used to prevent collisions across different resource detection steps.
 
 ### Detectors
 To make auto-detection implementations easy to use, the core resource package SHOULD define