On each heartbeat, master calls TSDescriptor methods like

  void set_num_sst_files (uint64_t num_sst_files) {
    std::lock_guard<decltype(lock_)> l(lock_);
    ts_metrics_.num_sst_files = num_sst_files;
  }

that takes a write lock, sets a field, and releases the write lock. To avoid doing this many times over for each individual field, create a method that takes a lock once and