feat(GCS+gRPC): timeout downloads in benchmark

[coryan]

This change is 

[google-cloud-cpp-bot]

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[codecov]

Codecov Report

Merging #7112 (04380cb) into main (c2e8c7a) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #7112   +/-   ##
=======================================
  Coverage   94.49%   94.49%           
=======================================
  Files        1309     1309           
  Lines      113020   113025    +5     
=======================================
+ Hits       106794   106802    +8     
+ Misses       6226     6223    -3     

Impacted Files	Coverage Δ
...orage/benchmarks/aggregate_throughput_benchmark.cc	87.95% <100.00%> (+0.06%)	⬆️
...storage/benchmarks/aggregate_throughput_options.cc	100.00% <100.00%> (ø)
...ge/benchmarks/aggregate_throughput_options_test.cc	100.00% <100.00%> (ø)
...bigtable/examples/bigtable_hello_instance_admin.cc	81.31% <0.00%> (-2.20%)	⬇️
.../cloud/storage/benchmarks/throughput_experiment.cc	74.87% <0.00%> (+0.50%)	⬆️
...le/cloud/storage/internal/curl_download_request.cc	89.15% <0.00%> (+1.20%)	⬆️
google/cloud/bigtable/internal/common_client.cc	97.14% <0.00%> (+1.42%)	⬆️

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[google-cloud-cpp-bot]

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For commit: 04380cbb893d543ca119a9b05575d09917b9e52d

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