Hashing improvements

[hoxxep]

As part of investigating whether rust-prometheus would benefit from rapidhash, I noticed a few improvements that could be made:

• The MetricVecCore::children hashmap uses keys that are hash values, so introducing NoHashHasher to avoid re-hashing this hash value improves performance.
• Allow with and get_metric_with to accept hashmaps with custom hashers.
• Introduce criterion's black_box in some hashmaps to reduce over-optimisation in microbenchmarks. Could be applied to further benchmarks.

Some of the counter.rs benchmarks are including hashmap initialisation and destruction. They are also using fixed values and small keys/values, which means benchmarks were unclear over whether FNV or rapidhash are beneficial (hash clashes, etc). Slightly longer key/value lengths are likely to benefit from rapidhash.

[ti-chi-bot]

Welcome @hoxxep! It looks like this is your first PR to tikv/rust-prometheus 🎉

[lucab]

Thanks for the PR!
I'd be happy to land something like this but I'm slightly wary about the state of https://github.com/paritytech/nohash-hasher.
Would you be open to drop nohash-hasher and directly implement a NoHasherU64 in here?

[hoxxep]

@lucab sure! I've removed the nohash-hasher dependency and had a sparring session with the DCO signoff bot, but that should be done now 👍

[lucab]

This generally looks fine, I only have a minor question.
It also needs a final cargo fmt --all in order to pass CI all checks.

[lucab]

I think these two can be more properly restricted to pub(crate), right?

[hoxxep]

Yeah. They're technically never re-exported, but I've adjusted to pub(crate) per your request so that it's explicit to the reader.

[lucab]

LGTM