async getByARR(value, currency = 'usd') {
        // find a milestone of the ARR type by a given value
        const milestone = await this.#MilestoneModel.findOne({type: 'arr', currency: currency, value: value}, {require: false});

        if (!milestone) {
            return null;
        }
        return this.#modelToMilestone(milestone);
    }
@param {number} value
@param {string} [currency]

@returns {Promise<Milestone|null>}