code: def get_post_agg(mconf):
          
          if mconf.get('type') == 'javascript':
              return JavascriptPostAggregator(
                  name=mconf.get('name', ''),
                  field_names=mconf.get('fieldNames', []),
                  function=mconf.get('function', ''))
          elif mconf.get('type') == 'quantile':
              return Quantile(
                  mconf.get('name', ''),
                  mconf.get('probability', ''),
              )
          elif mconf.get('type') == 'quantiles':
              return Quantiles(
                  mconf.get('name', ''),
                  mconf.get('probabilities', ''),
              )
          elif mconf.get('type') == 'fieldAccess':
              return Field(mconf.get('name'))
          elif mconf.get('type') == 'constant':
              return Const(
                  mconf.get('value'),
                  output_name=mconf.get('name', ''),
              )
          elif mconf.get('type') == 'hyperUniqueCardinality':
              return HyperUniqueCardinality(
                  mconf.get('name'),
              )
          elif mconf.get('type') == 'arithmetic':
              return Postaggregator(
                  mconf.get('fn', '/'),
                  mconf.get('fields', []),
                  mconf.get('name', ''))
          else:
              return CustomPostAggregator(
                  mconf.get('name', ''),
                  mconf)
docs: For a metric specified as `postagg` returns the
          kind of post aggregation for pydruid.