Switch to the official `prometheus-client` crate for metrics

[glindstedt]

We are currently using https://github.com/tikv/rust-prometheus but it seems like the future lies in https://github.com/prometheus/client_rust which is the official client.

Some more context:

• Enhancement proposals for rust-prometheus tikv/rust-prometheus#392 (comment)
• Is this project still maintained? tikv/rust-prometheus#530 (comment)