prombench: Uneven Load generation for `query_range`

[bwplotka]

Despite @bboreham improvements to loadgen (https://github.com/prometheus/test-infra/pull/618) we might need still some love to make the load exactly the same across two instances. For some reason it's uneven, and once uneven situation starts it persists through days:

link1

link2

This causes allocations, CPU use and HTTP latency metrics uncomparable across instances.

Acceptance Criteria

• Both instances receives exactly the same load, no matter how much fast they compute it.

[bwplotka]

Also I am curious if it makes sense to load different things in phases. e.g. 1h one API, 1h second API. Would allow us to isolate issues better.