impl(bigtable): add directpath metric

[scotthart]

This PR adds the client schema metric used to report the result of attempting to use DirectPath.

[gemini-code-assist]

Code Review

This pull request introduces the DirectAccessCompatibility metric and its associated labels to track Bigtable DirectPath compatibility, and updates MakeClientResourceLabels to extract instance and application profile details from options. Unit tests are also added to verify these changes. The review feedback recommends a performance optimization in IntoLabelMap to avoid unnecessary string copies and allocations on the hot path by directly emplacing labels instead of using a temporary array of structs.

[gemini-code-assist]

Creating a temporary array of structs with std::string members causes unnecessary copies and allocations of d.ip_preference and d.reason on every call to IntoLabelMap, which is on the hot path of recording metrics. Since there are only two labels to process, we can avoid the temporary array, the loop, and any unnecessary string copies by directly checking filtered_data_labels and emplacing the values.

  if (filtered_data_labels.find("ip_preference") == filtered_data_labels.end()) {
    labels.emplace("ip_preference", d.ip_preference);
  }
  if (filtered_data_labels.find("reason") == filtered_data_labels.end()) {
    labels.emplace("reason", d.reason);
  }

References

1. Scrutinize copies of non-fundamental C++ types. Is it necessary to copy the data? Can we move the data instead? ^(link)
2. When using a map with transparent lookups (e.g., absl::flat_hash_map<std::string, ...>), prefer find() over operator[] to avoid constructing a key object on every call. This is often more performant, especially if cache hits are common.

[scotthart]

Creation of temp structs removed.

[codecov]

Codecov Report

❌ Patch coverage is 99.12281% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.27%. Comparing base (30817a8) to head (2eb6e73).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...e/cloud/bigtable/internal/client_schema_metrics.cc	98.14%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16370      +/-   ##
==========================================
+ Coverage   92.26%   92.27%   +0.01%     
==========================================
  Files        2239     2239              
  Lines      210719   210816      +97     
==========================================
+ Hits       194412   194535     +123     
+ Misses      16307    16281      -26     

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[dbolduc]

nit: avoid default params for internal functions

[scotthart]

Done. A follow-up PR will update the .md files to help catch and prevent this type of style violation from occurring.

[dbolduc]

optional super nit:

I think if filtered_data_labels is a std::set<std::string, std::less<>> then we can have this fn take std::string_views as arguments, and only allocate the std::string if we need to emplace after the lookup.

[scotthart]

Good catch; done.