getMetric(name, scope) {
    return this._metrics.getMetric(name, scope)
  }
Look up a metric, and don't create it if it doesn't exist. Can create scopes
as a byproduct, but this function is only intended for use in testing, so
it's not a big deal.

@param {string} name Metric name.
@param {string} scope (Optional) The scope, if any, to which the metric
                      belongs.
@returns {object} Either a stats aggregate, an apdex stats aggregate, or
                 undefined.