_set(destinations, key, value, truncateExempt) {
    this.attributes[key] = { value, destinations, truncateExempt }
  }
Adds the given attribute to the instance attributes object,
overwriting existing keys if necessary.

@param {AttributeFilter.DESTINATIONS} destinations - Allowed destinations
@param {string}  key            - Attribute key
@param {string}  value          - Attribute value
@param {boolean} truncateExempt - Flag marking value exempt from truncation