feat: non-global metrics usage, add metrics sets

[Frando]

Description

• Allow to create structs implementing Metric without registering them to the superglobal Core
• Allow to optionally register these metrics to a prometheus_client::registry::Registry, and/or iterate over the individual metrics items manually
• Add a trait MetricSet which can be implemented on structs containing multiple structs that each implement Metric. It allows to iterate over metric structs, and each contained metrics item, and provides a method register_all to register all contained metrics to a prometheus registry.

This demonstrates how we could expose a struct EndpointMetrics from the endpoint, which has multiple metric sets inside, and still provide a straightforward way to both register them on a prometheus_client::Registry, and use them manually without prometheus_client.

Based on #12

Breaking Changes

Notes & open questions

Change checklist

• Self-review.
• Documentation updates following the style guide, if relevant.
• Tests if relevant.
• All breaking changes documented.

[github-actions]

Documentation for this PR has been generated and is available at: https://n0-computer.github.io/iroh-metrics/pr/14/docs/iroh_gossip/

Last updated: 2025-04-08T09:23:35Z

[Frando]

Closing in favor of #15