feat(common): make the RPC log even more readable

[devbww]

• Render google.protobuf.Duration and google.protobuf.Timestamp
  fields as descriptive strings rather than as raw numbers.

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This change is 

[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: 8c277115ca10972eb03d41ea314e404dcb2b81bc

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[codecov]

Codecov Report

Merging #9561 (363fcae) into main (71012ff) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #9561      +/-   ##
==========================================
- Coverage   94.37%   94.37%   -0.01%     
==========================================
  Files        1491     1491              
  Lines      137569   137611      +42     
==========================================
+ Hits       129829   129867      +38     
- Misses       7740     7744       +4     

Impacted Files	Coverage Δ
google/cloud/internal/log_wrapper.cc	83.16% <100.00%> (+6.14%)	⬆️
google/cloud/internal/log_wrapper_test.cc	99.47% <100.00%> (+0.04%)	⬆️
.../cloud/internal/streaming_read_rpc_logging_test.cc	95.55% <100.00%> (-0.10%)	⬇️
...cloud/internal/streaming_write_rpc_logging_test.cc	100.00% <100.00%> (ø)
...le/cloud/storage/internal/curl_download_request.cc	87.62% <0.00%> (-1.01%)	⬇️
...cloud/pubsub/internal/subscription_session_test.cc	97.98% <0.00%> (-0.68%)	⬇️
google/cloud/pubsub/subscriber_connection_test.cc	97.75% <0.00%> (-0.57%)	⬇️
google/cloud/bigtable/async_read_stream_test.cc	97.99% <0.00%> (+0.66%)	⬆️
...e/cloud/pubsublite/internal/alarm_registry_impl.cc	100.00% <0.00%> (+2.94%)	⬆️

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[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: 363fcae0750bb2ee5f3fd28e2fb4df728652287c

• Public Access Build Logs
• Cloud Build Console (requires auth)
• Kokoro Logs (requires auth)

ℹ️ NOTE: Kokoro logs are linked from "Details" below.