Support generics for MultiDimension APIs

[chenBright]

What problem does this PR solve?

Issue Number:

Problem Summary:

The current mbvar get/set operation APIs all require the construction of std::liststd::string, which requires the allocation of memory and the copying of strings, resulting in noticeable performance overhead.

What is changed and the side effects?

Changed:

The bvar get/set operation API supports more data structures through generics, thus achieving zero copy. Using Container<const char* / std::string_view / butil::StringPieces> can avoid string copying, where Container can be a container such as std::vector, std::set, std::map, etc. Furthermore, std::array and absl::InlinedVector can be used to avoid dynamic heap memory allocation.

Side effects:

• Performance effects:

• Breaking backward compatibility:

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Check List:

• Please make sure your changes are compilable.
• When providing us with a new feature, it is best to add related tests.
• Please follow Contributor Covenant Code of Conduct.

[Copilot]

Pull Request Overview

This PR refactors the MVariable API to support generic label container types and centralizes common static methods under a new MVariableBase class.

• Introduce MVariableBase for shared behavior and rename existing static methods (count_exposed, list_exposed, describe_exposed, dump_exposed) to live on it
• Convert MVariable into a template class parameterized by KeyType for label containers and update MultiDimension accordingly
• Update tests to exercise the new generic API and adjust CMake and consumers to call through MVariableBase

Reviewed Changes

Copilot reviewed 10 out of 11 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
test/bvar_mvariable_unittest.cpp	Switched static calls from MVariable to MVariableBase and added label‐container template tests
test/CMakeLists.txt	Added additional Darwin linker flags for new symbols
src/bvar/variable.cpp	Redirected dump_exposed call to MVariableBase
src/bvar/mvariable.h	Defined new MVariableBase, moved copy-control there, and templated MVariable on KeyType
src/bvar/mvariable.cpp	Updated all definitions to match MVariableBase signatures
src/bvar/multi_dimension.h & _inl.h	Templated MultiDimension on both T and KeyType, added hashing/equality functors
src/butil/strings/string_piece.h	Added C++17 std::string_view constructor and conversion operator
src/butil/containers/flat_map_inl.h	Removed commented-out logging
src/brpc/builtin/prometheus_metrics_service.cpp	Changed MVariable::dump_exposed calls to MVariableBase::dump_exposed

Comments suppressed due to low confidence (2)

src/bvar/multi_dimension_inl.h:384

• Typo in function name 'is_valid_lables_value'; consider renaming it to 'is_valid_labels_value' for clarity and consistency.

    os << "{\"name\" : \"" << this->name() << "\", \"labels\" : [";

src/bvar/multi_dimension_inl.h:386

• [nitpick] Enhance this error message by adding context and separators, e.g.:

LOG(ERROR) << "Invalid labels count: expected " << this->count_labels()
           << " but got " << labels_value.size();

    for (auto& label : this->_labels) {

[wwbmmm]

LGTM