class_docstr: Data manager for sampling data, use reservoir sampling.
      
code: def num_items_index(self):
      with self._mutex:
          return self._num_items_index
  
imports: import collections
  import random
  import threading
label_desc: Use image node_id 6 for calling the _ReservoirBucket obj's underlying member method code with example usage: obj.num_items_index() and returns: self