Prepare opentelemetry-prometheus for new protobuf version.

[hdost]

Originally posted by @dependabot in https://github.com/open-telemetry/opentelemetry-rust/pull/1548

We may even want to consider changing to the official Prometheus Rust crate. It didn't exist previously when the exporter was originally implemented.

• [ ] Consider switching to official crate.
• [ ] Determine the scope of the differences
• [ ] Verify the binary compatibility between the proto output
• [ ] Check if there's any user facing changes.

https://github.com/open-telemetry/opentelemetry-rust/blob/main/opentelemetry-prometheus/Cargo.toml#L26 Currently, exporter depends on the prometheus crate. This crate is not the official prometheus client.

This issue is opened to explore if we should switch to the official client library - https://crates.io/crates/prometheus-client , or maybe remove the dependency altogether.

From #1557

[cijothomas]

Where/why prometheus exporter needs protobuf ?

[lalitb]

The depedency is coming from prometheus client: https://github.com/tikv/rust-prometheus/blob/master/proto/proto_model.rs. Switching to prometheus-client should get rid of it.

[hdost]

I know the prometheus-client isn't currently as full featured. I can take a look.

[lalitb]

The prometheus client is not compatible with the latest protobuf version - https://github.com/tikv/rust-prometheus/blob/f49c724df0e123520554664436da68e555593af0/Cargo.toml#L30. So not sure if we can upgrade opentelemetry-prometheus with new protobuf version.

[cijothomas]

Related : https://github.com/open-telemetry/opentelemetry-rust/issues/1557