/// the same key.
  void Insert(const std::string& key, const T& value) {
    if (max_age_ == 0) {
      return;
    }
    absl::MutexLock lock(&mu_);
    InsertLocked(key, value);
  }

  // Delete the entry with key `key`. Return true if the entry was found for
  // `key`, false if the entry was not found. In both cases, there is no entry
  // with key `key` existed after the call.