test(pubub): add cleanup for pubsub topics

[suzmue]

Cleanup stale topics created by integration tests. Pubsub topics don't have a create time field, so instead create a label that records the creation time. There are restrictions on the characters used in labels including no uppercase letters. The wkt::Timestamp serialization uses uppercase letters, so instead this keeps it simple for the tests and stores the time in seconds.

Fixes #3355

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.21%. Comparing base (355c5ba) to head (753283a).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3363      +/-   ##
==========================================
+ Coverage   96.19%   96.21%   +0.02%     
==========================================
  Files         115      115              
  Lines        4574     4574              
==========================================
+ Hits         4400     4401       +1     
+ Misses        174      173       -1     

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