Make ECS detector not error when not on platform (#1428)

* Make ECS detector not error when not on platform

The OpenTelemetry specification states "failure to detect any resource
information MUST NOT be considered an error". Update the ECS detector to
not return an error when not running on the ECS platform.

Additionally, return nil instead of an empty resource to accommodate a
quick merge when this detector is used. A nil value means the other
resource this will be merged with will be returned immediately instead
of working through the actual merge.

* Add changes to changelog

* Match ECS detector tests with expected behavior

* Add PR number to changelog

* Remove unused errNotOnECS

CHANGELOG.md

@@ -8,6 +8,10 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 
 ## [Unreleased]
 
+### Fixed
+
+- The `"go.opentelemetry.io/contrib/detector/aws/ecs".Detector` no longer errors if not running in ECS. (#1426, #1428)
+
 ## [1.2.0/0.27.0] - 2021-11-15
 
 ### Changed

detectors/aws/ecs/ecs.go

@@ -40,7 +40,6 @@ var (
 	errCannotReadContainerID   = errors.New("failed to read container ID from cGroupFile")
 	errCannotReadContainerName = errors.New("failed to read hostname")
 	errCannotReadCGroupFile    = errors.New("ECS resource detector failed to read cGroupFile")
-	errNotOnECS                = errors.New("process is not on ECS, cannot detect environment variables from ECS")
 )
 
 // Create interface for methods needing to be mocked
@@ -74,7 +73,7 @@ func (detector *resourceDetector) Detect(ctx context.Context) (*resource.Resourc
 	metadataURIV4 := os.Getenv(metadataV4EnvVar)
 
 	if len(metadataURIV3) == 0 && len(metadataURIV4) == 0 {
-		return empty, errNotOnECS
+		return nil, nil
 	}
 	hostName, err := detector.utils.getContainerName()
 	if err != nil {

detectors/aws/ecs/ecs_test.go

@@ -106,6 +106,7 @@ func TestReturnsIfNoEnvVars(t *testing.T) {
 	detector := &resourceDetector{utils: nil}
 	res, err := detector.Detect(context.Background())
 
-	assert.Equal(t, errNotOnECS, err)
-	assert.Equal(t, 0, len(res.Attributes()))
+	// When not on ECS, the detector should return nil and not error.
+	assert.NoError(t, err, "failure to detect when not on platform must not be an error")
+	assert.Nil(t, res, "failure to detect should return a nil Resource to optimize merge")
 }