Method get_hit_summary

example/use-async-logger/example.cpp:70–73

      return cached_allocator.free_page_num();
    }
    static ::bvar::Stat get_hit_summary(void*) {
      auto summary = cached_allocator.cache_hit_summary();
      return ::bvar::Stat {summary.sum, static_cast<ssize_t>(summary.num)};
    }
  };
  static ::bvar::PassiveStatus<size_t> pending_size {
      "test-babylon-pending", S::get_pending_size, nullptr};