docs(pubsub): add into_stream example

[dbolduc]

Part of the work for #4508

Add an example for MessageStream::into_stream().

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.94%. Comparing base (ad17bcf) to head (19e074f).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4965      +/-   ##
==========================================
+ Coverage   93.93%   93.94%   +0.01%     
==========================================
  Files         221      221              
  Lines        8538     8538              
==========================================
+ Hits         8020     8021       +1     
+ Misses        518      517       -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.