Streaming PromQL engine: benchmarking improvements

[charleskorn]

What this PR does

This PR makes two small changes to the benchmarking setup for the streaming PromQL engine:

• The Prometheus engine is now referred to consistently by that name, rather than sometimes as the "standard" engine
• tools/benchmark-query-engine can now start the ingester used for benchmarks and run it continuously, so multiple invocations of tools/benchmark-query-engine can use the same ingester. This means we can avoid waiting to load data into the ingester on each run of tools/benchmark-query-engine, which takes about a minute on my machine.

Which issue(s) this PR fixes or relates to

(none)

Checklist

• [n/a] Tests updated.
• [n/a] Documentation added.
• [n/a] CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX].
• [n/a] about-versioning.md updated with experimental features.

[jhesketh]

lgtm
(the capitalisation of Prometheus and not streaming bothers me, but I understand why :-p)