addAttributes(destinations, attrs) {
    for (const key in attrs) {
      if (properties.hasOwn(attrs, key)) {
        this.addAttribute(destinations, key, attrs[key])
      }
    }
  }
Passthrough method for adding multiple unknown attributes at once.

@param {DESTINATIONS}  destinations  - The default destinations for these attributes.
@param {object}        attrs         - The attributes to add.