refactor(bigtable): rename metric Labels types to indicate Table schema

[scotthart]

With the upcoming introduction of Client schema metric labels, this helps differentiate.

[gemini-code-assist]

Code Review

This pull request renames ResourceLabels to TableResourceLabels and DataLabels to TableDataLabels across the Bigtable internal metrics implementation, updating the clone method signatures to accept these structures by const reference instead of by value. Two issues were identified in the review: first, changing Metric::clone from a pure virtual function to a virtual function returning nullptr reduces compile-time safety and could lead to runtime null pointer dereferences; second, ApplicationBlockingLatency::clone incorrectly attempts to std::move the const& parameters, which is a no-op and misleading.

[codecov]

Codecov Report

❌ Patch coverage is 97.10611% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.24%. Comparing base (c6a56cb) to head (570a1a6).

Files with missing lines	Patch %	Lines
...oud/bigtable/internal/operation_context_factory.cc	84.74%	9 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16337   +/-   ##
=======================================
  Coverage   92.24%   92.24%           
=======================================
  Files        2227     2227           
  Lines      209208   209241   +33     
=======================================
+ Hits       192976   193007   +31     
- Misses      16232    16234    +2     

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