Replace prometheus client with the official one

[ackintosh]

Description

Prometheus team have released an official Prometheus instrumentation library for Rust.

https://github.com/prometheus/client_rust

• announcement: https://twitter.com/PrometheusIO/status/1483584441419276293
• background: Enhancement proposals for rust-prometheus tikv/rust-prometheus#392 (comment)

We're not having any problems with the current client for now, however, I believe we would take benefits from the official one. ref: tikv/rust-prometheus#392

[ackintosh]

In lighthouse, the client is used to:

1. provide metrics endpoint for Prometheus HTTP server, in OpenMetrics specification.

• beacon_node/http_metrics crate
• validator_client/http_metrics crate

2. send BeaconNode/Validator metrics to custom endpoint (e.g. beaconcha.in), in "JSON".

• monitoring_api crate

The metrics is defined as Ethereum consensus client metrics and Beacon chain metrics.

[mxinden]

👋 libp2p and prometheus-client maintainer here.

Another benefit would be dependency consolidation. libp2p uses prometheus-client and in turn the lighthouse network stack uses prometheus-client.

[ackintosh]

📝 I'm working on prometheus/client_rust#47. This makes the client able to do our the 2. use case I've mentioned here.

[ackintosh]

📝 I found out that lighthouse is using HistogramTimer, which is timer to measure and record the duration of an event. (e.g. here)

For now, if I understand correctly, the official client doesn’t have a functionality equivalent to HistogramTimer.

[mxinden]

I find HistogramTimer a need feature of the prometheus crate. I think it is worth proposing it to the prometheus-client crate @ackintosh.