static validateHostnames(hostnames) {
    if (Array.isArray(hostnames) && hostnames.every(host => typeof host === 'string')) {
      return hostnames.map(Budget.validateHostname);
    } else if (hostnames !== undefined) {
      throw new Error(`firstPartyHostnames should be defined as an array of strings.`);
    }
  }
@param {unknown} hostnames
@return {undefined|Array<string>}