impl(bigtable): duplicate AsyncRowSampler

[dbolduc]

The ground work for #9173

cp AsyncRowSampler LegacyAsyncRowSampler

After this PR, I will modify AsyncRowSampler to use the modern Stub instead of the DataClient.

REVIEWER: this is broken up into multiple commits to make the changes easier to see.

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This change is 

[google-cloud-cpp-bot]

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[codecov]

Codecov Report

Merging #9221 (1917aa0) into main (d2aad2b) will decrease coverage by 0.00%.
The diff coverage is 97.05%.

@@            Coverage Diff             @@
##             main    #9221      +/-   ##
==========================================
- Coverage   94.52%   94.51%   -0.01%     
==========================================
  Files        1454     1454              
  Lines      130338   130338              
==========================================
- Hits       123198   123192       -6     
- Misses       7140     7146       +6     

Impacted Files	Coverage Δ
google/cloud/bigtable/data_client.h	100.00% <ø> (ø)
google/cloud/bigtable/table.cc	99.25% <ø> (ø)
...bigtable/internal/legacy_async_row_sampler_test.cc	96.34% <96.34%> (ø)
...loud/bigtable/internal/legacy_async_row_sampler.cc	100.00% <100.00%> (ø)
google/cloud/pubsub/subscriber_connection_test.cc	97.22% <0.00%> (-0.70%)	⬇️
google/cloud/spanner/database_admin_connection.cc	85.14% <0.00%> (-0.58%)	⬇️
...le/cloud/spanner/database_admin_connection_test.cc	99.40% <0.00%> (-0.30%)	⬇️
google/cloud/pubsub/samples/samples.cc	91.97% <0.00%> (-0.08%)	⬇️
.../cloud/bigtable/internal/async_row_sampler_test.cc
...oogle/cloud/bigtable/internal/async_row_sampler.cc
... and 1 more

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[devbww]

What's the point of "restore AsyncRowSampler but don't build it"?

[dbolduc]

What's the point of "restore AsyncRowSampler but don't build it"?

My thinking was that the reviewer can easily verify that

1. the async_row_sampler* code didn't change (by looking at the overall diff of the PR)
2. the legacy_async_row_sampler* code is in use

[devbww]

So, you'll remove it in a followup?

[dbolduc]

So, you'll remove it in a followup?

I will transform AsyncRowSampler to use the "modern API" (Stub classes, common policies) instead of the bigtable specific ones. Then I will start building it again.

The LegacyAsyncRowSampler will exist for nearly a year until the deprecation policy expires on #8800

[devbww]

I will transform AsyncRowSampler to use the "modern API" (Stub classes, common policies) instead of the bigtable specific ones. Then I will start building it again.

I see. Thanks.