getStopReason(event) {
    if (this.stopReasonKey.includes('.')) {
      const parts = this.stopReasonKey.split('.')
      let val = event
      for (const p of parts) {
        val = val[p]
      }
      return val
    }

    return event[this.stopReasonKey]
  }
Finds the reason for the end of the stream based upon the model's known
stop reason key.

@param {object} event
@returns {string}