fix(testing_util): link opentelemetry-cpp SDK as PUBLIC

[GaetanLepage]

Problem

google/cloud/testing_util/opentelemetry_matchers.h is a public header of the google_cloud_cpp_testing target, and it includes and uses opentelemetry-cpp SDK symbols inline (e.g. opentelemetry::sdk::trace::SpanData::GetAttributes()).

Any test that links google_cloud_cpp_testing and includes that header (e.g. google/cloud/internal/opentelemetry_test.cc) therefore references those SDK symbols directly from its own translation unit.

The CMake build, however, links the SDK targets as PRIVATE:

target_link_libraries(
    google_cloud_cpp_testing
    PUBLIC absl::symbolize absl::failure_signal_handler google-cloud-cpp::common
           GTest::gmock
    PRIVATE opentelemetry-cpp::in_memory_span_exporter opentelemetry-cpp::trace)

So they are not propagated to the test executables. When building with shared libraries (BUILD_SHARED_LIBS=ON), linking the test fails:

undefined reference to symbol
  '...opentelemetry::v1::sdk::trace::SpanData::GetAttributes[abi:cxx11]() const'
libopentelemetry_trace.so: error adding symbols: DSO missing from command line

because libopentelemetry_trace.so only arrives as an indirect DT_NEEDED of libgoogle_cloud_cpp_testing.so, which modern binutils refuses to resolve symbols from.

Fix

Move opentelemetry-cpp::in_memory_span_exporter and opentelemetry-cpp::trace to the PUBLIC link interface, since they are part of the public (header) interface of google_cloud_cpp_testing.

Consistency with Bazel

The Bazel build already exposes these dependencies transitively: :google_cloud_cpp_testing depends on :google_cloud_cpp_testing_private, which lists @opentelemetry-cpp//exporters/memory:in_memory_span_exporter and @opentelemetry-cpp//sdk/src/trace in deps. This change aligns the CMake build with the existing Bazel behavior.

Discovered while packaging google-cloud-cpp for nixpkgs (shared-library build with tests enabled).

[gemini-code-assist]

Code Review

This pull request updates the google_cloud_cpp_testing library in google/cloud/testing_util/CMakeLists.txt by moving the opentelemetry-cpp dependencies from PRIVATE to PUBLIC visibility. The reviewer suggested formatting the target_link_libraries list to have one dependency per line to improve readability and simplify future version control diffs.

[gemini-code-assist]

For better readability and to minimize future diffs, consider listing each dependency on its own line in target_link_libraries. This is a common practice in this repository for maintaining long lists of dependencies.

           GTest::gmock
           opentelemetry-cpp::in_memory_span_exporter
           opentelemetry-cpp::trace

[GaetanLepage]

Thanks. I kept the dependencies packed rather than one-per-line because that's what this repository's .cmake-format.py config produces (and cmake-format is enforced in CI). The pre-existing PUBLIC absl::symbolize absl::failure_signal_handler google-cloud-cpp::common ... line was already packed for the same reason. I ran cmake-format -c .cmake-format.py so the block now matches the canonical formatting; switching to one dependency per line would fail the format check.