Computes the alert rule's metric over the last 30.44 days (2,629,746,000 milliseconds before now), with the alert rule's filters applied, and returns it as a bare number. The unit is the metric's own: USD for TOTAL_COST and COST_PER_TRANSACTION, milliseconds for DURATION, tokens for the token count metrics, and a count for ERROR_COUNT. For an alert rule whose alertType is THRESHOLD, on TOTAL_COST, a token count or DURATION it is the largest value of a single transaction in that window (the smallest when the operator is less than); otherwise it is the window's total. Two exceptions: COST_PER_TRANSACTION is always the average cost per transaction in USD, and TOKENS_PER_MINUTE is the selected token value divided by the window length in seconds. Metric types this call does not support return an error.