Merge pull request #29730 from backstage/freben/once

update documentation to register metrics once

docs/tutorials/setup-opentelemetry.md

@@ -27,6 +27,11 @@ yarn --cwd packages/backend add \
 In your `packages/backend/src` folder, create an `instrumentation.js` file.
 
 ```typescript title="in packages/backend/src/instrumentation.js"
+// Prevent from running more than once (due to worker threads)
+if (!require('node:worker_threads').isMainThread) {
+  return;
+}
+
 const { NodeSDK } = require('@opentelemetry/sdk-node');
 const {
   getNodeAutoInstrumentations,

packages/backend/src/instrumentation.js

@@ -14,6 +14,11 @@
  * limitations under the License.
  */
 
+// Prevent from running more than once (due to worker threads)
+if (!require('node:worker_threads').isMainThread) {
+  return;
+}
+
 const { NodeSDK } = require('@opentelemetry/sdk-node');
 const {
   getNodeAutoInstrumentations,