PromBench: execute longer queries to check speed of data from disk blocks

[bboreham]

PromBench runs a set of queries to generate fake load, both instant and range queries. The longest range is 1 hour, so it runs entirely in the head block. It would be more realistic to have some longer queries, say 4 hours or 24 hours.

However, the data used by queries is built up during the PromBench run, so a 24-hour query would be mostly empty until PromBench had been running for many hours. It would be better to store or pre-generate some blocks of data so Prometheus has 24h (or longer) of data to query immediately.