docstr: Maximum possible byte gain from merging this into another
  characteristic.
code: def get_gain(self):
      """Maximum possible byte gain from merging this into another
      characteristic."""
      count = len(self.items)
      return max(0, self.overhead - count)
  
imports: from fontTools.misc.roundTools import noRound, otRound
  from fontTools.misc.intTools import bit_count
  from fontTools.ttLib.tables import otTables
  from fontTools.varLib.models import supportScalar
  from fontTools.varLib.builder import buildVarRegionList, buildVarStore, buildVarRegion, buildVarData
  from functools import partial
  from collections import defaultdict
  from heapq import heappush, heappop
label_desc: Use image node_id 5 for calling the _Encoding obj's underlying member method code with example usage: obj.get_gain() and returns: max