[arvados-dispatch-cloud] Cumulative instance time and cost metrics
In principle, these metrics can be estimated based on the existing instances_total and instances_price metrics. However:
- (counter) cumulative instance time and cost, partitioned by allocation state and node type
- prometheus doesn't offer an integral function
- updating a cumulative metric after every worker/pool update (i.e., in updateMetrics()) would provide decent accuracy even when prometheus sampling is infrequent/unreliable.