function getExclusiveDurationInMillis(trace) {
  if (this._exclusiveDuration == null) {
    // Calculate the exclusive time for the subtree rooted at `this`
    const calculator = new ExclusiveCalculator(this, trace)
    calculator.process()
  }
  return this._exclusiveDuration
}
The duration of the transaction trace tree that only this level accounts
for.

@returns {integer} The amount of time the trace took, minus any child
                  segments, in milliseconds.