feat(bigtable): add explicit instance MakeDataConnection

[scotthart]

This pull request introduces a new overload of MakeDataConnection that accepts a vector of InstanceResource objects to encourage explicit instance declaration during client initialization, enabling dynamic channel pooling and channel priming. Instances not initially named can be used, but will incur additionally latency on first use as a dynamic channel pool is created for that instance and priming is skipped in favor of immediately sending the RPC.

[gemini-code-assist]

Code Review

This pull request introduces a new overload of MakeDataConnection that accepts a vector of InstanceResource objects to encourage explicit instance declaration during client initialization, enabling dynamic channel pooling. The previous overload has been deprecated, and examples, benchmarks, and tests have been updated accordingly. The review feedback suggests using std::move on Options objects passed to MakeDataConnection to avoid unnecessary copies, aligning with the repository's style guide.

[codecov]

Codecov Report

❌ Patch coverage is 77.62238% with 32 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.23%. Comparing base (0a8f509) to head (6db9498).
⚠️ Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
...ogle/cloud/bigtable/tests/data_integration_test.cc	14.28%	24 Missing ⚠️
google/cloud/bigtable/examples/data_snippets.cc	72.72%	6 Missing ⚠️
...loud/bigtable/examples/bigtable_examples_common.cc	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16191      +/-   ##
==========================================
- Coverage   92.24%   92.23%   -0.01%     
==========================================
  Files        2265     2265              
  Lines      210126   210218      +92     
==========================================
+ Hits       193832   193898      +66     
- Misses      16294    16320      +26     

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[dbolduc]

unrelated?

[scotthart]

This is to help alert users of the new overload that they're sending RPCs to an instance that was not named at time of construction. This may be anticipated behavior for some applications, but may prompt other applications to update the instances named when creating the DataConnection.

[dbolduc]

same

[dbolduc]

comment: deprecating an API that 100% of customers use seems like a big push for an optional feature.

I am sure @igorbernstein2 told you to do it though.

[scotthart]

Replacing the existing fixed connection pool with the dynamic pool is less of an optional feature and more of an evolution of the library. The dynamic pool has performance benefits from channel priming and is more efficient with resources. Deprecating the existing MakeDataConnection helps provide "better defaults" when using the library.