fix(instr-fetch): do not enable in Node.js; clarify in docs this inst…

…r is for web fetch only (#4498)

* fix(instr-fetch): do not enable in Node.js; clarify in docs this instr is for web fetch only

* add a changelog entry

* add a diagnostic warning if attempting to use instr-fetch in Node.js

* fixup! add a diagnostic warning if attempting to use instr-fetch in Node.js

---------

Co-authored-by: Marc Pichler <marc.pichler@dynatrace.com>

experimental/CHANGELOG.md

@@ -26,6 +26,7 @@ All notable changes to experimental packages in this project will be documented
 * fix(instrumentation): normalize paths for internal files in scoped packages [#4467](/~https://github.com/open-telemetry/opentelemetry-js/pull/4467) @pichlermarc
   * Fixes a bug where, on Windows, internal files on scoped packages would not be instrumented.
 * fix(otlp-transformer): only use BigInt inside hrTimeToNanos() [#4484](/~https://github.com/open-telemetry/opentelemetry-js/pull/4484) @pichlermarc
+* fix(instrumentation-fetch): do not enable in Node.js; clarify in docs this instr is for web fetch only [#4498](/~https://github.com/open-telemetry/opentelemetry-js/pull/4498) @trentm
 
 ### :books: (Refine Doc)
 

experimental/packages/opentelemetry-instrumentation-fetch/README.md

@@ -5,7 +5,8 @@
 
 **Note: This is an experimental package under active development. New releases may include breaking changes.**
 
-This module provides auto instrumentation for web using fetch.
+This module provides auto instrumentation for web using [fetch](https://developer.mozilla.org/en-US/docs/Web/API/fetch).
+(Note: This instrumentation does **not** instrument [Node.js' fetch](https://nodejs.org/api/globals.html#fetch). See [this issue](/~https://github.com/open-telemetry/opentelemetry-js/issues/4333).)
 
 ## Installation
 

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

@@ -35,6 +35,8 @@ import { _globalThis } from '@opentelemetry/core';
 // safe enough
 const OBSERVER_WAIT_TIME_MS = 300;
 
+const isNode = typeof process === 'object' && process.release?.name === 'node';
+
 export interface FetchCustomAttributeFunction {
   (
     span: api.Span,
@@ -465,6 +467,14 @@ export class FetchInstrumentation extends InstrumentationBase<
    * implements enable function
    */
   override enable(): void {
+    if (isNode) {
+      // Node.js v18+ *does* have a global `fetch()`, but this package does not
+      // support instrumenting it.
+      this._diag.warn(
+        "this instrumentation is intended for web usage only, it does not instrument Node.js's fetch()"
+      );
+      return;
+    }
     if (isWrapped(fetch)) {
       this._unwrap(_globalThis, 'fetch');
       this._diag.debug('removing previous patch for constructor');
@@ -476,6 +486,9 @@ export class FetchInstrumentation extends InstrumentationBase<
    * implements unpatch function
    */
   override disable(): void {
+    if (isNode) {
+      return;
+    }
     this._unwrap(_globalThis, 'fetch');
     this._usedResources = new WeakSet<PerformanceResourceTiming>();
   }