add pip install flask step

Author: yiyuan-he

.github/workflows/release-udp-exporter.yml

@@ -45,6 +45,10 @@ jobs:
         run: |
           pip install ./exporters/aws-otel-otlp-udp-exporter/dist/*.whl
 
+      - name: Install Flask
+        run: |
+          pip install flask
+
       - name: Run Sample App in Background
         working-directory: sample-applications/integ-test-app
         run: |