httpSafe() {
    if (!this.base64Payload) {
      this.base64Payload = Buffer.from(this.plainTextPayload, 'utf-8').toString('base64')
    }
    logger.trace('DistributedTracePayload httpSafe: %s', this.base64Payload)
    return this.base64Payload
  }
Construct a payload suitable for HTTP transport.

@returns {string} The base64 encoded JSON representation of the
distributed trace payload.