[Fixed] Wait until worker shutdown is complete

[crimoniv]

🎩 What?

When shutting down workers, ensure proper termination by waiting until the future is complete (with future.result()).

This should fix a spammy error we're getting on each shutdown:

Observed terminating stream error Cancelled('Locally cancelled by application!')

This started happening in google-cloud-pubsub==2.31.0 (changelog), most probably due to the inclusion of this fix.

🤔 Why?

.result() is expected to be called after .cancel() when shutting down, according to the documentation.

We're already following this approach when safely bootstrapping subscription consumers.

🔗 Related issue

N/A