getOrCreateMetric(name, scope) {
    return this._metrics.getOrCreateMetric(name, scope)
  }
Look up the mapping from a name (and optionally a scope) to a set of metric
data for that name, creating the data if they don't already exist.

@param {string} name The name of the requested metric.
@param {string} scope (Optional) The scope to which the metric is bound.
@returns {Stats} The aggregated data for that name.