class_docstr: A map-to-arrays dict, with deterministic Reservoir Sampling.
  
  Store each reservoir bucket by key, and each bucket is a list sampling
  with reservoir algorithm.
docstr: Add a new item to reservoir buckets with given tag as key.
  
  For usage habits of VisualDL, actually call self._add_items()
  
  Args:
      run: Identity of one tablet.
      tag: Identity of one record in tablet.
      item: New item to add to bucket.
docstr_tok: ["Add","a","new","item","to","reservoir","buckets","with","given","tag","as","key",".","For","usage","habits","of","VisualDL",",","actually","call","self._add_items","(",")","Args",":","run",":","Identity","of","one","tablet",".","tag",":","Identity","of","one","record","in","tablet",".","item",":","New","item","to","add","to","bucket","."]
code: def add_item(self, run, tag, item):
      """Add a new item to reservoir buckets with given tag as key.
  
      For usage habits of VisualDL, actually call self._add_items()
  
      Args:
          run: Identity of one tablet.
          tag: Identity of one record in tablet.
          item: New item to add to bucket.
      """
      key = run + "/" + tag
      self._add_item(key, item)
  
imports: import collections
  import random
  import threading
label_desc: Use image node_id 12 for calling the Reservoir obj's underlying member method code with example usage: obj.add_item(run, tag, item) without return types