The added zlib dependency is for the bundled prometheus-cpp, else:

CMake Error at .../cmake-3.18/.../share/cmake-3.18/Modules/FindPackageHandleStandardArgs.cmake:165 (message):
  Could NOT find ZLIB (missing: ZLIB_LIBRARY ZLIB_INCLUDE_DIR)
Call Stack (most recent call first):
  .../cmake-3.18/.../share/cmake-3.18/Modules/FindPackageHandleStandardArgs.cmake:458 (_FPHSA_FAILURE_MESSAGE)
  .../cmake-3.18/.../share/cmake-3.18/Modules/FindZLIB.cmake:115 (FIND_PACKAGE_HANDLE_STANDARD_ARGS)
  pull/CMakeLists.txt:9 (find_package)