fix(opentelemetry-instrumentation-fetch): fixed override of headers

packages/opentelemetry-instrumentation-fetch/src/fetch.ts

@@ -158,6 +158,10 @@ export class FetchInstrumentation extends InstrumentationBase<
       api.propagation.inject(api.context.active(), options.headers, {
         set: (h, k, v) => h.set(k, typeof v === 'string' ? v : String(v)),
       });
+    } else if(options.headers instanceof Headers) {
+      api.propagation.inject(api.context.active(), options.headers, {
+        set: (h, k, v) => h.set(k, typeof v === 'string' ? v : String(v)),
+      });
     } else {
       const headers: Partial<Record<string, unknown>> = {};
       api.propagation.inject(api.context.active(), headers);

packages/opentelemetry-instrumentation-fetch/test/fetch.test.ts

@@ -535,6 +535,32 @@ describe('fetch', () => {
       assert.ok(typeof r.headers.get(X_B3_TRACE_ID) === 'string');
     });
 
+    it('should keep custom headers with a request object and a headers object', () => {
+      const r = new Request('url', {
+        headers: new Headers({'foo': 'bar'})
+      });
+      window.fetch(r).catch(() => {});
+      assert.ok(r.headers.get('foo') === 'bar');
+    });
+
+    it('should keep custom headers with url, untyped request object and typed headers object', () => {
+      const url = 'url';
+      const init = {
+        headers: new Headers({'foo': 'bar'})
+      };
+      window.fetch(url, init).catch(() => {});
+      assert.ok(init.headers.get('foo') === 'bar');
+    });
+
+    it('should keep custom headers with url, untyped request object and untyped headers object', () => {
+      const url = 'url';
+      const init = {
+        headers: {'foo': 'bar'}
+      };
+      window.fetch(url, init).catch(() => {});
+      assert.ok(init.headers['foo'] === 'bar');
+    });
+
     it('should pass request object as first parameter to the original function (#2411)', () => {
       const r = new Request(url);
       return window.fetch(r).then(() => {