feat(instrumentation-fetch): Add applyCustomAttributesOnSpan option

packages/opentelemetry-instrumentation-fetch/README.md

@@ -61,6 +61,14 @@ fetch('http://localhost:8090/fetch.js');
 
 See [examples/tracer-web/fetch](https://github.com/open-telemetry/opentelemetry-js/tree/main/examples/tracer-web) for a short example.
 
+### Fetch Instrumentation options
+
+Fetch instrumentation plugin has few options available to choose from. You can set the following:
+
+| Options                                                                                                                                           | Type                          | Description                           |
+| ------------------------------------------------------------------------------------------------------------------------------------------------- | ----------------------------- | ------------------------------------- |
+| [`applyCustomAttributesOnSpan`](https://github.com/open-telemetry/opentelemetry-js/blob/main/packages/opentelemetry-instrumentation-fetch/src/fetch.ts#L47) | `HttpCustomAttributeFunction` | Function for adding custom attributes |
+
 ## Useful links
 
 - For more information on OpenTelemetry, visit: <https://opentelemetry.io/>

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

@@ -19,6 +19,7 @@ import {
   isWrapped,
   InstrumentationBase,
   InstrumentationConfig,
+  safeExecuteInTheMiddle,
 } from '@opentelemetry/instrumentation';
 import * as core from '@opentelemetry/core';
 import * as web from '@opentelemetry/web';
@@ -43,6 +44,14 @@ const getUrlNormalizingAnchor = () => {
   return a;
 };
 
+export interface FetchCustomAttributeFunction {
+  (
+    span: api.Span,
+    request: Request | RequestInit,
+    result: Response | FetchError
+  ): void;
+}
+
 /**
  * FetchPlugin Config
  */
@@ -61,6 +70,8 @@ export interface FetchInstrumentationConfig extends InstrumentationConfig {
    * also not be traced.
    */
   ignoreUrls?: Array<string | RegExp>;
+  /** Function for adding custom attributes on the span */
+  applyCustomAttributesOnSpan?: FetchCustomAttributeFunction;
 }
 
 /**
@@ -311,6 +322,7 @@ export class FetchInstrumentation extends InstrumentationBase<
           response: Response
         ) {
           try {
+            plugin._applyAttributesAfterFetch(span, options, response);
             if (response.status >= 200 && response.status < 400) {
               plugin._endSpan(span, spanData, response);
             } else {
@@ -331,6 +343,7 @@ export class FetchInstrumentation extends InstrumentationBase<
           error: FetchError
         ) {
           try {
+            plugin._applyAttributesAfterFetch(span, options, error);
             plugin._endSpan(span, spanData, {
               status: error.status || 0,
               statusText: error.message,
@@ -360,6 +373,28 @@ export class FetchInstrumentation extends InstrumentationBase<
     };
   }
 
+  private _applyAttributesAfterFetch(
+    span: api.Span,
+    request: Request | RequestInit,
+    result: Response | FetchError
+  ) {
+    const applyCustomAttributesOnSpan = this._getConfig()
+      .applyCustomAttributesOnSpan;
+    if (applyCustomAttributesOnSpan) {
+      safeExecuteInTheMiddle(
+        () => applyCustomAttributesOnSpan(span, request, result),
+        error => {
+          if (!error) {
+            return;
+          }
+
+          api.diag.error('applyCustomAttributesOnSpan', error);
+        },
+        true
+      );
+    }
+  }
+
   /**
    * Prepares a span data - needed later for matching appropriate network
    *     resources

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

@@ -35,7 +35,11 @@ import {
 } from '@opentelemetry/web';
 import * as assert from 'assert';
 import * as sinon from 'sinon';
-import { FetchInstrumentation, FetchInstrumentationConfig } from '../src';
+import {
+  FetchInstrumentation,
+  FetchInstrumentationConfig,
+  FetchCustomAttributeFunction,
+} from '../src';
 import { AttributeNames } from '../src/enums/AttributeNames';
 import { SemanticAttributes } from '@opentelemetry/semantic-conventions';
 
@@ -57,6 +61,7 @@ const getData = (url: string, method?: string) =>
     },
   });
 
+const CUSTOM_ATTRIBUTE_KEY = 'span kind';
 const defaultResource = {
   connectEnd: 15,
   connectStart: 13,
@@ -576,6 +581,73 @@ describe('fetch', () => {
     });
   });
 
+  describe('applyCustomAttributesOnSpan option', () => {
+    const noop = () => {};
+    const prepare = (
+      url: string,
+      applyCustomAttributesOnSpan: FetchCustomAttributeFunction,
+      cb: VoidFunction = noop
+    ) => {
+      const propagateTraceHeaderCorsUrls = [url];
+
+      prepareData(cb, url, {
+        propagateTraceHeaderCorsUrls,
+        applyCustomAttributesOnSpan,
+      });
+    };
+
+    afterEach(() => {
+      clearData();
+    });
+
+    it('applies attributes when the request is succesful', done => {
+      prepare(
+        url,
+        span => {
+          span.setAttribute(CUSTOM_ATTRIBUTE_KEY, 'custom value');
+        },
+        () => {
+          const span: tracing.ReadableSpan = exportSpy.args[1][0][0];
+          const attributes = span.attributes;
+
+          assert.ok(attributes[CUSTOM_ATTRIBUTE_KEY] === 'custom value');
+          done();
+        }
+      );
+    });
+
+    it('applies custom attributes when the request fails', done => {
+      prepare(
+        badUrl,
+        span => {
+          span.setAttribute(CUSTOM_ATTRIBUTE_KEY, 'custom value');
+        },
+        () => {
+          const span: tracing.ReadableSpan = exportSpy.args[1][0][0];
+          const attributes = span.attributes;
+
+          assert.ok(attributes[CUSTOM_ATTRIBUTE_KEY] === 'custom value');
+          done();
+        }
+      );
+    });
+
+    it('has request and response objects in callback arguments', done => {
+      const applyCustomAttributes: FetchCustomAttributeFunction = (
+        span,
+        request,
+        response
+      ) => {
+        assert.ok(request.method === 'GET');
+        assert.ok(response.status === 200);
+
+        done();
+      };
+
+      prepare(url, applyCustomAttributes);
+    });
+  });
+
   describe('when url is ignored', () => {
     beforeEach(done => {
       const propagateTraceHeaderCorsUrls = url;