docs: def new_relic_stats(self):
          """
          Get stats for this instance.
          """
          if self._new_relic_stats is None:
              # if this is a sharded instance, fetch shard stats in parallel
              if self.type == 'mongodb_sharded':
                  shards = [Shard(self.name, self._service_url + 'shards/',
                                  self._client, shard_doc)
                            for shard_doc in self.shards().get('data')]
                  fs = []
                  with futures.ThreadPoolExecutor(len(shards)) as executor:
                      for shard in shards:
                          fs.append(executor.submit(shard.get_shard_stats))
                      futures.wait(fs, timeout=None, return_when=futures.ALL_COMPLETED)
                  stats_this_second = self._rollup_shard_stats_to_instance_stats(
                      {shard.name: future.result() for (shard, future) in zip(shards, fs)})
                  # power nap
                  time.sleep(1)
                  # fetch again
                  fs = []
                  with futures.ThreadPoolExecutor(len(shards)) as executor:
                      for shard in shards:
                          fs.append(executor.submit(shard.get_shard_stats))
                      futures.wait(fs, timeout=None, return_when=futures.ALL_COMPLETED)
                  stats_next_second = self._rollup_shard_stats_to_instance_stats(
                      {shard.name: future.result() for (shard, future) in zip(shards, fs)})
                  self._new_relic_stats = self._compile_new_relic_stats(stats_this_second, stats_next_second)
              else:
                  # fetch stats like we did before (by hitting new_relic_stats API resource)
                  response = requests.get('{}{}'.format(self._url,
                                          'new-relic-stats'),
                                          **self._instances._default_request_kwargs)
                  self._new_relic_stats = json.loads(response.content).get(
                      'data') if response.status_code == 200 else {}
          return self._new_relic_stats