feat(@opentelemetry-instrumentation-fetch): support reading response body from the hook applyCustomAttributesOnSpan (#2497)

Co-authored-by: Valentin Marchaud <contact@vmarchaud.fr>

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

@@ -331,6 +331,7 @@ export class FetchInstrumentation extends InstrumentationBase<
         ): void {
           try {
             const resClone = response.clone();
+            const resClone4Hook = response.clone();
             const body = resClone.body;
             if (body) {
               const reader = body.getReader();
@@ -338,7 +339,7 @@ export class FetchInstrumentation extends InstrumentationBase<
                 reader.read().then(
                   ({ done }) => {
                     if (done) {
-                      endSpanOnSuccess(span, response);
+                      endSpanOnSuccess(span, resClone4Hook);
                     } else {
                       read();
                     }

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

@@ -683,6 +683,22 @@ describe('fetch', () => {
 
       prepare(url, applyCustomAttributes);
     });
+
+    it('get response body from callback arguments response', done => {
+      const applyCustomAttributes: FetchCustomAttributeFunction = async (
+        span,
+        request,
+        response
+      ) => {
+        if(response instanceof Response ){
+          const rsp = await response.json();
+          assert.deepStrictEqual(rsp.args, {});
+          done();
+        }
+      };
+
+      prepare(url, applyCustomAttributes);
+    });
   });
 
   describe('when url is ignored', () => {