Let's go with option elimination
A. Port the application code to run on Google App Engine
>> PaaS serverless managed service, so all my infra provisioning is taken care by GCP.
B. Integrate Cloud Dataflow into the application to capture real-time metrics
>> Good to have
C. Instrument the application with a monitoring tool like Stackdriver Debugger
>> Is a must for debugging issues and monitoring application logs this is now GCP Cloud monitoring and logging.
D. Select an automation framework to reliably provision the cloud infrastructure
>> App Engine is a PaaS so the infrastructure is taken care of by App Engine, I would select this if I have not selected A, hence will eliminate this option for now
E. Deploy a continuous integration tool with automated testing in a staging environment
>> Good to have
A and C are must-have
B and E are Good to have, but E has more importance than B
Hence will go with ACE.