class_docstr: Metaclass for PlotlyCallback classes.
  
  We want each callback class to keep track of all of the instances of the class.
  Using a meta class here lets us keep the logic for instance tracking in one place.
code: def __call__(cls, *args, **kwargs):
      inst = super().__call__(*args, **kwargs)
  
      # Store weak reference to the callback instance in the _instances
      # WeakValueDictionary. This will allow instances to be garbage collected and
      # the references will be automatically removed from the collection when this
      # happens.
      cls.instances[inst.plot.trace_uid] = inst
  
      return inst
  
imports: from weakref import WeakValueDictionary
  from ...element import Tiles
  from ...streams import BoundsX, BoundsXY, BoundsY, RangeX, RangeXY, RangeY, Selection1D, SelectionXY, Stream
  from .util import _trace_to_subplot
label_desc: Use image node_id 2 for calling the PlotlyCallbackMetaClass obj's underlying member method code with example usage: obj.__call__(cls) and returns: inst