async function getVariables(key, serviceId = null) {
  const where = {
    name: key,
    service_id: serviceId,
  };

  const variables = await db.Variable.findAll({
    attributes: ['value', 'user_id'],
    where,
  });

  return variables.map((variable) => variable.get({ plain: true }));
}
@description Get all variables matching with key.
@param {string} key - The unique key of the variable.
@param {string} [serviceId] - The unique ID of a service, or null.
@returns {Promise<Array>} Resolves with list of variables.
@example
gladys.user.getVariables();