Specify result of Resource.merge error (#1726)

The specification currently states that merging Resources and aggregating
ResourceDetectors results in an error, but does not say anything about the
resource with which the program continues to operate. This change specifies
that the resultant resource of these methods is undefined and the decision is
left to each implementation.

Author: dgetu

CHANGELOG.md

@@ -42,6 +42,7 @@ release.
 ### Resource
 
 - Add schema_url support to `Resource`. ([#1692](https://github.com/open-telemetry/opentelemetry-specification/pull/1692))
+- Clarify result of Resource merging and ResourceDetector aggregation in case of error. ([#1726](https://github.com/open-telemetry/opentelemetry-specification/pull/1726))
 
 ### Semantic Conventions
 

specification/resource/sdk.md

@@ -82,7 +82,8 @@ The resulting resource will have the Schema URL calculated as follows:
 - Else if the Schema URLs of the old and updating resources are the same then
   that will be the Schema URL of the resulting resource,
 - Else this is a merging error (this is the case when the Schema URL of the old
-  and updating resources are not empty and are different).
+  and updating resources are not empty and are different). The resulting resource is
+  undefined, and its contents are implementation-specific.
 
 Required parameters:
 
@@ -124,7 +125,8 @@ a semantic convention or if the detector does not know what attributes it will
 populate (e.g. the detector that reads the attributes from environment values
 will not know what Schema URL to use). If multiple detectors are combined and
 the detectors use different non-empty Schema URL it MUST be an error since it is
-impossible to merge such resources.
+impossible to merge such resources. The resulting resource is undefined, and its
+contents are implementation specific.
 
 ### Specifying resource information via an environment variable