message: Follow best practice on acquiring multiple locks
diff: @@ -39,8 +39,9 @@ metric_registry::~metric_registry() {
  void metric_registry::merge(metric_registry& other) {
  if (this == &other)
  return;
  - std::unique_lock<std::mutex> guard1{families_mx_};
  - std::unique_lock<std::mutex> guard2{other.families_mx_};
  + std::unique_lock<std::mutex> guard1{families_mx_, std::defer_lock};
  + std::unique_lock<std::mutex> guard2{other.families_mx_, std::defer_lock};
  + std::lock(guard1, guard2);
  families_.reserve(families_.size() + other.families_.size());
  for (auto& fptr : other.families_)
  if (fetch(fptr->prefix(), fptr->name()) != nullptr)
  