fix(configuration): remove default propagator initialization (#6399)

Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: MikeGoldsmith

experimental/CHANGELOG.md

@@ -21,6 +21,7 @@ For notes on migrating to 2.x / 0.200.x see [the upgrade guide](doc/upgrade-to-2
 
 ### :bug: Bug Fixes
 
+* fix(configuration): remove default propagator initialization  [#6399](https://github.com/open-telemetry/opentelemetry-js/pull/6399) @MikeGoldsmith
 * fix(instrumentation-fetch): preserve Response.url, type, and redirected properties [#6243](https://github.com/open-telemetry/opentelemetry-js/issues/6243) @AnubhavPurohit691
   * The fetch instrumentation now preserves the read-only `url`, `type`, and `redirected` properties from the original Response object when wrapping it with a Proxy. This fixes issues where code relying on these properties (e.g., CORS type detection) would fail with instrumented fetch.
 * fix(exporter-prometheus): add missing `@opentelemetry/semantic-conventions` dependency [#6330](https://github.com/open-telemetry/opentelemetry-js/pull/6330) @omizha

experimental/packages/configuration/src/FileConfigFactory.ts

@@ -287,6 +287,9 @@ export function setPropagator(
     );
     if (compositeListString) {
       config.propagator!.composite_list = compositeListString;
+    } else if (auxList.length > 0) {
+      // Generate composite_list from the composite entries
+      config.propagator!.composite_list = auxList.join(',');
     }
   }
 }

experimental/packages/configuration/src/models/configModel.ts

@@ -81,10 +81,6 @@ export function initializeDefaultConfiguration(): ConfigurationModel {
     attribute_limits: {
       attribute_count_limit: 128,
     },
-    propagator: {
-      composite: [{ tracecontext: null }, { baggage: null }],
-      composite_list: 'tracecontext,baggage',
-    },
   };
 
   return config;

experimental/packages/configuration/test/ConfigFactory.test.ts

@@ -53,10 +53,7 @@ const defaultConfig: ConfigurationModel = {
   attribute_limits: {
     attribute_count_limit: 128,
   },
-  propagator: {
-    composite: [{ tracecontext: null }, { baggage: null }],
-    composite_list: 'tracecontext,baggage',
-  },
+  propagator: {},
 };
 
 const defaultTracerProvider: TracerProvider = {
@@ -1112,6 +1109,14 @@ describe('ConfigFactory', function () {
       assert.deepStrictEqual(configFactory.getConfigModel(), expectedConfig);
     });
 
+    it('should not set propagators by default', function () {
+      const configFactory = createConfigFactory();
+      const config = configFactory.getConfigModel();
+      assert.deepStrictEqual(config, defaultConfig);
+      assert.strictEqual(config.propagator?.composite, undefined);
+      assert.strictEqual(config.propagator?.composite_list, undefined);
+    });
+
     it('should return config with custom propagator', function () {
       process.env.OTEL_PROPAGATORS = 'tracecontext,jaeger';
       const expectedConfig: ConfigurationModel = {
@@ -2102,7 +2107,11 @@ describe('ConfigFactory', function () {
         'test/fixtures/short-config.yml';
       const configFactory = createConfigFactory();
       const expectedConfig: ConfigurationModel = {
-        ...defaultConfig,
+        disabled: false,
+        log_level: DiagLogLevel.INFO,
+        attribute_limits: {
+          attribute_count_limit: 128,
+        },
         resource: {
           attributes_list: 'service.instance.id=123',
           attributes: [
@@ -2350,7 +2359,11 @@ describe('ConfigFactory', function () {
         'test/fixtures/resources.yaml';
       const configFactory = createConfigFactory();
       const expectedConfig: ConfigurationModel = {
-        ...defaultConfig,
+        disabled: false,
+        log_level: DiagLogLevel.INFO,
+        attribute_limits: {
+          attribute_count_limit: 128,
+        },
         resource: {
           schema_url: 'https://opentelemetry.io/schemas/1.16.0',
           attributes_list:
@@ -2470,7 +2483,10 @@ describe('ConfigFactory', function () {
       config = {};
       setPropagator(config, { composite: [{ tracecontext: null }] });
       assert.deepStrictEqual(config, {
-        propagator: { composite: [{ tracecontext: null }] },
+        propagator: {
+          composite: [{ tracecontext: null }],
+          composite_list: 'tracecontext',
+        },
       });
 
       const res = getTemporalityPreference(