impl(generator): add OperationContext to Bigtable stub and decorators

[scotthart]

As part of ongoing Bigtable metric collection, some additional metrics require data to be collected in a stub decorator. To enable this, we need to plumb the bigtable_internal::OperationContext object into the decorators via the stub RPC methods. This requires a minor but widespread change to how we emit stubs from the generator. For now, we are going to limit this generator change to the bigtable library where it is needed. In the future, as we expand metric collection, we will factor OperationContext out into a common library and make the generator changes service generic instead of specific to bigtable.

The code changes have been grouped into separate commits for clarity.

[gemini-code-assist]

Code Review

This pull request introduces an experimental feature to propagate OperationContext through the Bigtable client stub and decorator layers. It adds a new configuration option experimental_bigtable_operation_context to the code generator, updates the C++ code generator templates (including auth, logging, metadata, round-robin, stub, and tracing decorators) to support passing this context, and integrates the context parameter across the handwritten Bigtable client implementation files and their corresponding unit tests. I have no feedback to provide as there are no review comments.

[codecov]

Codecov Report

❌ Patch coverage is 71.80617% with 128 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.23%. Comparing base (95b11e1) to head (88a60f9).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...loud/bigtable/internal/bigtable_channel_refresh.cc	2.85%	34 Missing ⚠️
generator/internal/descriptor_utils.cc	41.66%	14 Missing ⚠️
generator/internal/tracing_stub_generator.cc	0.00%	13 Missing ⚠️
generator/internal/metadata_decorator_generator.cc	0.00%	12 Missing ⚠️
generator/internal/auth_decorator_generator.cc	0.00%	10 Missing ⚠️
...erator/internal/round_robin_decorator_generator.cc	0.00%	10 Missing ⚠️
generator/internal/stub_generator.cc	9.09%	10 Missing ⚠️
generator/internal/stub_generator_base.cc	0.00%	9 Missing ⚠️
generator/internal/logging_decorator_generator.cc	0.00%	7 Missing ⚠️
...oud/bigtable/internal/data_connection_impl_test.cc	95.79%	5 Missing ⚠️
... and 1 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16331      +/-   ##
==========================================
- Coverage   92.24%   92.23%   -0.01%     
==========================================
  Files        2227     2227              
  Lines      209169   209208      +39     
==========================================
+ Hits       192943   192969      +26     
- Misses      16226    16239      +13     

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