refactor: Add derive macro and Metric trait

[Frando]

Description

• Adds a Metric trait that is implemented on Gauge and Counter to provide a common interface.
• Adds a custom trait and derive macro Iterable that works similar to struct_iterable, but returns an iterator over &dyn Metric and does not allocate for iteration.
• Adds a derive macro MetricsGroup that expands to the Iterable impl and also implements Default by calling FieldType::new(description) for each field, where description is the first line of the field's doc comment, or a custom string set via #[metrics(description = "my description")]. Also implements MetricsGroup, with a name of either the struct name converted to camel_case, or a custom name set via #[metrics_group(name = "my_name")]
• Simplifies the iterators over MetricsGroup and MetricsGroupSet by using the new Metric trait and the dyn field iterator from the derived Iterable impl
• Removes the MetricDescription types, those are not needed anymore.

Breaking Changes

• struct_iterable::Iterable is no longer exported from iroh-metrics, and is no longer a supertrait of MetricsGroup
• instead, MetricsGroup now has iroh_metrics::Iterable and iroh_metrics::IntoIterable supertraits. They can be implemented manually, but it is recommended to use the new MetricsGroup or Iterable derive macros exported from iroh_metrics. See their docs for details.
• MetricsGroupSet::iter is renamed to MetricsGroupSet::groups
• MetricsGroup::values and MetricsGroup::describe are removed. Use MetricsGroup::iter instead.
• MetricItem struct changed, but still provides access to the same data, through the name() method and its deref to &dyn Metric.
• MetricDescription is removed, everything there is now accessible over MetricItem through MetricsGroup::iter.

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/18/docs/iroh_gossip/

Last updated: 2025-04-11T09:40:08Z

[Arqu]

Working through macros is always slightly mind bending.
Thanks for the tests, makes it so much easier to parse and understand.
👍