fix: specification compliant resource collision precedence (#1975)

Co-authored-by: Daniel Dyla <dyladan@users.noreply.github.com>
Co-authored-by: Bartlomiej Obecny <bobecny@gmail.com>
Co-authored-by: Valentin Marchaud <contact@vmarchaud.fr>

README.md

@@ -245,6 +245,12 @@ To request automatic tracing support for a module not on this list, please [file
 
 ## Upgrade guidelines
 
+### 0.17.0 to 0.18.0
+
+[PR-1975](https://github.com/open-telemetry/opentelemetry-js/pull/1975)
+
+- Breaking change - The resulting resource MUST have all attributes that are on any of the two input resources. If a key exists on both the old and updating resource, the value of the updating resource MUST be picked - previously it was opposite.
+
 ### 0.16.0 to 0.17.0
 
 [PR-1880](https://github.com/open-telemetry/opentelemetry-js/pull/1880) feat(diag-logger): introduce a new global level api.diag for internal diagnostic logging

packages/opentelemetry-resources/src/Resource.ts

@@ -54,7 +54,7 @@ export class Resource {
 
   /**
    * Returns a new, merged {@link Resource} by merging the current Resource
-   * with the other Resource. In case of a collision, current Resource takes
+   * with the other Resource. In case of a collision, other Resource takes
    * precedence.
    *
    * @param other the Resource that will be merged with this.
@@ -66,8 +66,8 @@ export class Resource {
     // SpanAttributes from resource overwrite attributes from other resource.
     const mergedAttributes = Object.assign(
       {},
-      other.attributes,
-      this.attributes
+      this.attributes,
+      other.attributes
     );
     return new Resource(mergedAttributes);
   }

packages/opentelemetry-resources/test/Resource.test.ts

@@ -50,7 +50,7 @@ describe('Resource', () => {
 
   it('should return merged resource when collision in attributes', () => {
     const expectedResource = new Resource({
-      'k8s.io/container/name': 'c1',
+      'k8s.io/container/name': 'c2',
       'k8s.io/namespace/name': 'default',
       'k8s.io/pod/name': 'pod-xyz-123',
       'k8s.io/location': 'location1',