my rpm package contains an extra prometheus directory

[whhe]

I include the prometheus-cpp in my project, the cmake file is like below:

include(ExternalProject)
set(PROMETHEUS_INSTALL_DIR ${THIRD_PARTY_PATH}/install/prometheus-cpp)

if (NOT EXISTS ${PROMETHEUS_INSTALL_DIR}/include)
    execute_process(COMMAND mkdir -p ${PROMETHEUS_INSTALL_DIR}/include COMMAND_ERROR_IS_FATAL ANY)
endif ()
add_library(prometheus-cpp INTERFACE)
ExternalProject_Add(
    extern_prometheus-cpp
    ${EXTERNAL_PROJECT_LOG_ARGS}
    GIT_REPOSITORY "https://github.com/jupp0r/prometheus-cpp.git"
    GIT_TAG "v1.1.0"
    PREFIX ${THIRD_PARTY_PATH}/prometheus-cpp
    CMAKE_ARGS
    -DCMAKE_POSITION_INDEPENDENT_CODE=ON
    -DCMAKE_C_COMPILER=${CMAKE_C_COMPILER}
    -DCMAKE_CXX_FLAGS=${CMAKE_CXX_FLAGS}
    -DCMAKE_CXX_FLAGS_RELEASE=${CMAKE_CXX_FLAGS_RELEASE}
    -DCMAKE_CXX_FLAGS_DEBUG=${CMAKE_CXX_FLAGS_DEBUG}
    -DCMAKE_C_FLAGS=${CMAKE_C_FLAGS}
    -DCMAKE_INSTALL_PREFIX=${PROMETHEUS_INSTALL_DIR}
    -DENABLE_PUSH=ON
    -DENABLE_PULL=ON
    -DENABLE_TESTING=OFF
    -DENABLE_COMPRESSION=ON
    -DUSE_THIRDPARTY_LIBRARIES=ON
)
add_dependencies(prometheus-cpp extern_prometheus-cpp)
target_link_libraries(prometheus-cpp INTERFACE
        ${PROMETHEUS_INSTALL_DIR}/lib64/libprometheus-cpp-pull.a
        ${PROMETHEUS_INSTALL_DIR}/lib64/libprometheus-cpp-push.a
        ${PROMETHEUS_INSTALL_DIR}/lib64/libprometheus-cpp-core.a
)
target_include_directories(prometheus-cpp INTERFACE ${PROMETHEUS_INSTALL_DIR}/include)

I use CPack to make the rpm for my project, but I find that there is an extra directory $PROMETHEUS_INSTALL_DIR in the rpm. What could be the reason?