update documentation to register metrics once

Signed-off-by: Fredrik Adelöw <freben@gmail.com>

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,