PromBench Idea: exercise more parts of PromQL

[bboreham]

PromBench runs synthetic load defined in 6_loadgen.yaml. These cover selectors, rate, histogram_quantile and sum, but leave out most of PromQL.

For instance we could add:

• binary arithmetic operators (joins)
• logical operators (and, or, unless)
• topk

[HarshitNagpal29]

@bboreham PTAL https://github.com/prometheus/test-infra/pull/723