async getCurrentMrr() {
        const knex = this.knex;
        const rows = await knex('members_stripe_customers_subscriptions')
            .select(knex.raw(`plan_currency as currency`))
            .select(knex.raw(`SUM(mrr) AS mrr`))
            .groupBy('plan_currency')
            .orderBy('currency');

        if (rows.length === 0) {
            // Add a USD placeholder to always have at least one currency
            rows.push({
                currency: 'usd',
                mrr: 0
            });
        }

        return rows;
    }
Get the current total MRR, grouped by currency (ascending order)
@returns {Promise<MrrByCurrency[]>}