[google-cloud-cpp] fix opentelemetry feature

[dbolduc]

cc @coryan - I goofed the original PR.

I only verified that opentelemetry-cpp was being installed as a dep. I did not verify that google_cloud_cpp_opentelemetry was installed.

Previously, google_cloud_cpp_opentelemetry was not listed as a target nor installed.

Now:

$ ./vcpkg install google-cloud-cpp[core,experimental-opentelemetry]
# <omitted>
google-cloud-cpp provides CMake targets:

    # this is heuristically generated, and may not be correct
    find_package(google_cloud_cpp_common CONFIG REQUIRED)
    target_link_libraries(main PRIVATE google-cloud-cpp::common)

    find_package(google_cloud_cpp_googleapis CONFIG REQUIRED)
    # note: 74 additional targets are not displayed.
    target_link_libraries(main PRIVATE gRPC::grpc gRPC::grpc++ google-cloud-cpp::iam_protos google-cloud-cpp::pubsub_protos)

    find_package(google_cloud_cpp_grpc_utils CONFIG REQUIRED)
    target_link_libraries(main PRIVATE google-cloud-cpp::grpc_utils)

    find_package(google_cloud_cpp_opentelemetry CONFIG REQUIRED)
    target_link_libraries(main PRIVATE google-cloud-cpp::experimental-opentelemetry)

    find_package(google_cloud_cpp_rest_internal CONFIG REQUIRED)
    target_link_libraries(main PRIVATE google-cloud-cpp::rest_internal)

    find_package(google_cloud_cpp_trace CONFIG REQUIRED)
    target_link_libraries(main PRIVATE google-cloud-cpp::trace google-cloud-cpp::trace_protos)

$ ls installed/x64-linux/share/ | grep google_cloud_cpp
google_cloud_cpp_common
google_cloud_cpp_googleapis
google_cloud_cpp_grpc_utils
google_cloud_cpp_opentelemetry
google_cloud_cpp_rest_internal
google_cloud_cpp_trace

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