Extend Prometheus storage interface to do multiple queries in parallel, and use that to dedupe index lookups.

[tomwilkie]

We could notice when we're doing overlapping queries (for instance, fetching all chunk ids for a metric vs some subset) and do the filtering client side (ie

(count(max(node_cpu{job="monitoring/prom-node-exporter"}) by (cpu, node)) by (node) - sum(irate(node_cpu{job="monitoring/prom-node-exporter",mode="idle"}[1m])) by (node)) 

    / 

count(max(node_cpu{job="monitoring/prom-node-exporter"}) by (cpu, node)) by (node) * 100

fetches node_cpu{job="monitoring/prom-node-exporter"} and node_cpu{job="monitoring/prom-node-exporter",mode="idle"}, the later could be filtered client side)