fix: missing .fields() method in jaeger propagator (#268)

Author: jtmalinowski

packages/opentelemetry-propagator-jaeger/src/JaegerHttpTracePropagator.ts

@@ -80,6 +80,10 @@ export class JaegerHttpTracePropagator implements TextMapPropagator {
 
     return setExtractedSpanContext(context, spanContext);
   }
+
+  fields(): string[] {
+    return [this._jaegerTraceHeader];
+  }
 }
 
 /**

packages/opentelemetry-propagator-jaeger/test/JaegerHttpTracePropagator.test.ts

@@ -172,6 +172,19 @@ describe('JaegerHttpTracePropagator', () => {
     });
   });
 
+  describe('.fields()', () => {
+    it('returns the default header if not customized', () => {
+      assert.deepStrictEqual(jaegerHttpTracePropagator.fields(), [
+        'uber-trace-id',
+      ]);
+    });
+    it('returns the customized header if customized', () => {
+      assert.deepStrictEqual(customJaegerHttpTracePropagator.fields(), [
+        customHeader,
+      ]);
+    });
+  });
+
   it('should fail gracefully on bad responses from getter', () => {
     const ctx1 = jaegerHttpTracePropagator.extract(
       ROOT_CONTEXT,