text: /**
   * Interface for custom context propagation.  If extract or inject methods are present they
   * will be used in the propagation chain.
   */
  export default class Propagator {
    /**
     * Extract span context from a given carrier.
     * @param {object} carrier
     * @return {SpanContext}
     */
    extract (carrier) {}
  
    /**
     * Inject span context into a given carrier.
     * @param {SpanContext} spanContext
     * @param {object} carrier
     */
    inject (spanContext, carrier) {}
  }
  