try out other prometheus client libraries

[clux]

The tikv one barely gets updates, has shown no activity on any of my previous issues / prs, so I am skeptical it will ever support exmplars/openmetrics or newer features.

• [x] official prometheus/client_rust for exemplar support and opementrics format - #72
• [x] conrad's measured for perf and typesafe apis - #71

[clux]

Have done a bit of testing of two competing metrics crates (the ones i would use personally):

• measured in #71
• promethus_client in #72

The end result is not massively different in terms of code (despite measured's performance focus), local rebuild times (touch metrics.rs && cargo build with and without release) are both identical on my machine, and still both marginally faster than the current tikv crate, which was kind of surprising with their proc macro usage.

Unclear which one to go with as of yet, or if I have missed something important. The official one is lacking in terms of maintenance and sophistication, and measured is slightly complex and lacks exemplars. If the official one had more maintenance I might like that more, but currently leaning towards helping measured getting some better ergonomics in the edge places.