feat: support logging for unified Rest credentials

[coryan]

Fixes #10316 and fixes #3429

This change is 

[google-cloud-cpp-bot]

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[codecov]

Codecov Report

Base: 93.86% // Head: 93.85% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (61c9881) compared to base (f601caa).
Patch coverage: 61.11% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10412      +/-   ##
==========================================
- Coverage   93.86%   93.85%   -0.01%     
==========================================
  Files        1610     1610              
  Lines      145810   145821      +11     
==========================================
- Hits       136860   136859       -1     
- Misses       8950     8962      +12     

Impacted Files	Coverage Δ
...cloud/storage/internal/unified_rest_credentials.cc	72.22% <33.33%> (-1.37%)	⬇️
google/cloud/internal/unified_rest_credentials.cc	73.58% <57.14%> (-1.42%)	⬇️
...ogle/cloud/internal/oauth2_decorate_credentials.cc	80.00% <75.00%> (-20.00%)	⬇️
...bigtable/examples/bigtable_hello_instance_admin.cc	81.00% <0.00%> (-2.00%)	⬇️
...le/cloud/storage/internal/curl_download_request.cc	89.29% <0.00%> (-0.67%)	⬇️
...cloud/pubsub/internal/subscription_session_test.cc	97.99% <0.00%> (-0.67%)	⬇️
google/cloud/completion_queue_test.cc	97.13% <0.00%> (-0.20%)	⬇️
google/cloud/internal/credentials_impl.h	100.00% <0.00%> (ø)
google/cloud/pubsub/subscriber_connection_test.cc	100.00% <0.00%> (+1.21%)	⬆️

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[devbww]

Perhaps this could also do with the "if needed" restriction.

"If needed" might also be expanded to something like "if requested by the Options" in both cases.

[google-cloud-cpp-bot]

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