code: def _ProcessHistogram(self, tag, wall_time, step, histo):
      
      histo = self._ConvertHistogramProtoToTuple(histo)
      histo_ev = HistogramEvent(wall_time, step, histo)
      self.histograms.AddItem(tag, histo_ev)
      self.compressed_histograms.AddItem(tag, histo_ev, self._CompressHistogram)
docs: Processes a proto histogram by adding it to accumulated state.