Specification of Metricbeat/statsd field mappings

I have metricbeat + statsd collecting metrics of the form a.b.c.d. When they show up in kibana, they have been converted to statsd.a_b_c_d.count (counter) or statsd.a_b_c_d.value (gauge). I would like to break the metric label into fields a, b, c, and d, but it's not clear this is possible with the statsd.mappings metricbeat configuration. I've tried a number of variations, none of which has the desired results (in all cases, the metric label is unchanged and no fields are mapped):

statsd.mappings:
  # With statsd prefix
  - metric: statsd.<a>.<b>.<c>.<d>.count  # multi- substitution (original label)
  - metric: statsd.<a>_<b>_<c>_<d>.count  # multi-substitution
  - metric: statsd.a.<rest>.count  # single substitution (original label)
  - metric: statsd.a_<rest>.count  # single substitution
  # w/o statsd prefix
  - metric: <a>_<b>_<c>_<d>.count 
  - metric: <a>.<b>.<c>.<d>.count
  - metric: a_<rest>.count
  - metric: a.<rest>.count

The examples suggest at least one of these options should work.