OpenTelemetry Protocol support for metrics

[Dubzer]

recently, support for the Prometheus Pushgateway was added in #1404. while it allows for easier migration from resticprofile, there is now a standard that doesn't require you to deploy a Pushgateway and is supported by several metrics-focused databases out of the box, including the Prometheus itself. i suggest adding OTLP support and marking it as a recommended solution in the docs.

there's an SDK for Rust that can be used to accomplish this: https://github.com/open-telemetry/opentelemetry-rust

if this is something you would like to see in the project, I can try to contribute it