fix(api): add public 'fields' function to api.propagator

packages/opentelemetry-api/src/api/propagation.ts

@@ -96,6 +96,16 @@ export class PropagationAPI {
     return this._getGlobalPropagator().extract(context, carrier, getter);
   }
 
+  /**
+   * Return a list of all fields which may be used by the propagator.
+   *
+   * This list should be used to clear fields before calling inject if a carrier is
+   * used more than once.
+   */
+  public fields(): string[] {
+    return this._getGlobalPropagator().fields();
+  }
+
   /** Remove the global propagator */
   public disable() {
     delete _global[GLOBAL_PROPAGATION_API_KEY];

packages/opentelemetry-api/test/api/api.test.ts

@@ -150,6 +150,13 @@ describe('API', () => {
         assert.strictEqual(data.carrier, carrier);
         assert.strictEqual(data.getter, getter);
       });
+
+      it('fields', () => {
+        api.propagation.setGlobalPropagator(new TestTextMapPropagation());
+
+        const fields = api.propagation.fields();
+        assert.deepStrictEqual(fields, ['TestField']);
+      });
     });
   });
 });