[observability/resource] fix service name calculation (#3204)

* fix service name calculation

* drop comment

* fix linter

* simplify comment

observability/resource/default.go

@@ -17,6 +17,8 @@ limitations under the License.
 package resource
 
 import (
+	"os"
+
 	"knative.dev/pkg/changeset"
 	"knative.dev/pkg/system"
 
@@ -25,6 +27,8 @@ import (
 	semconv "go.opentelemetry.io/otel/semconv/v1.34.0"
 )
 
+const otelServiceNameKey = "OTEL_SERVICE_NAME"
+
 // Default returns a default OpenTelemetry Resource enriched
 // with common Knative/Kubernetes attributes.
 //
@@ -33,10 +37,16 @@ import (
 // - PodName using system.PodName
 // - ServiceVersion with changeset.Get
 func Default(serviceName string) *resource.Resource {
+	// If the OTEL_SERVICE_NAME is set then let this override
+	// our own serviceName
+	if name := os.Getenv(otelServiceNameKey); name != "" {
+		serviceName = name
+	}
+
 	attrs := []attribute.KeyValue{
 		semconv.K8SNamespaceName(system.Namespace()),
-		semconv.ServiceName(serviceName),
 		semconv.ServiceVersion(changeset.Get()),
+		semconv.ServiceName(serviceName),
 	}
 
 	if pn := system.PodName(); pn != "" {
@@ -46,13 +56,11 @@ func Default(serviceName string) *resource.Resource {
 	// Ignore the error because it complains about semconv
 	// schema version differences
 	resource, err := resource.Merge(
+		resource.Default(),
 		resource.NewWithAttributes(
 			semconv.SchemaURL,
 			attrs...,
 		),
-		// We merge 'Default' last since this allows overriding
-		// the service name etc. using env variables
-		resource.Default(),
 	)
 	if err != nil {
 		panic(err)

observability/resource/default_test.go

@@ -19,6 +19,9 @@ package resource
 import (
 	"testing"
 
+	"github.com/google/go-cmp/cmp"
+	semconv "go.opentelemetry.io/otel/semconv/v1.34.0"
+
 	_ "knative.dev/pkg/system/testing"
 )
 
@@ -28,4 +31,35 @@ func TestDefault(t *testing.T) {
 	if r == nil {
 		t.Fatalf("expected resource to not be nil")
 	}
+
+	for _, v := range r.Attributes() {
+		if v.Key == semconv.ServiceNameKey {
+			got := v.Value.AsString()
+			want := "myservice"
+
+			if diff := cmp.Diff(want, got); diff != "" {
+				t.Error("unexpected service name diff (-want +got):", diff)
+			}
+		}
+	}
+}
+
+func TestDefaultWithEnvOverride(t *testing.T) {
+	t.Setenv(otelServiceNameKey, "another")
+	r := Default("myservice")
+
+	if r == nil {
+		t.Fatalf("expected resource to not be nil")
+	}
+
+	for _, v := range r.Attributes() {
+		if v.Key == semconv.ServiceNameKey {
+			got := v.Value.AsString()
+			want := "another"
+
+			if diff := cmp.Diff(want, got); diff != "" {
+				t.Error("unexpected service name diff (-want +got):", diff)
+			}
+		}
+	}
 }