def get_field_value(self, key, default=None):
    if key in self._fields:
      payload_field = self._fields[key]
      if payload_field:
        return payload_field.value
    return default
time_unit is the period of time on which the moving average is computed,
dataset/ETHPy150Open pantsbuild/pants/src/python/pants/base/payload.py/Payload.get_field_value--crossover with--dataset/ETHPy150Open avalente/appmetrics/appmetrics/meter.py/EWMA.__init__