feat(metrics): add support for metric families and label serialisation

[ethanolchik]

Overview

This PR builds on the already-merged scalar metrics #232 (Counter, Gauge) to add labelled metrics. It introduces a serde-based system for serialising metric label sets into the Prometheus protobuf data model, and adds a Family abstraction for grouping metrics by their label values.

Label serialisation and error handling

• Introduced a new labels module with:
  • to_label_pairs: serialises any serde::Serialize label set into protobuf LabelPair values.
  • A serde Serializer implementation that flattens a label struct into name/value pairs, rejecting unsupported compound types (sequences, maps, nested structs, byte arrays) with clear errors.
  • LabelError: a dedicated error type implementing serde::ser::Error.
• Preserves the scalar formatting behaviour of the existing metrics backend: ryu-based float formatting, serde renames, enum variants, optional values, and Display adapters.
• Validates label names against [a-zA-Z_:][a-zA-Z0-9_:]*.
• Stores label values as raw strings - OpenMetrics escaping is deliberately deferred to the text encoder, keeping the protobuf model format-independent.
• Added serde (with derive), ryu, and parking_lot dependencies to support label serialisation and efficient concurrent metric storage.

Metrics API enhancements

• Added Family<S, M, C>: shared, labelled metric storage with:
  • get_or_create, remove, and clear lifecycle operations; clones share the same underlying metrics.
  • A MetricConstructor trait (with a blanket Fn() -> M impl) for metrics that need configuration at creation time.
• Fail-soft collection: when encoding, invalid label sets and metric groups with inconsistent metadata are skipped (with a diagnostic) rather than failing the entire scrape; multi-series metrics (e.g. RangeGauge) are grouped correctly by suffix.
• Re-exported Family, MetricConstructor, LabelError, to_label_pairs, and set_collect_error_hook from the crate root for downstream use.

Decoupling logging from metrics

• Collection diagnostics are emitted through an installable hook (set_collect_error_hook) rather than a hard-coded logger, defaulting to stderr when none is set.
• Follow-up (for when foundations adopts this crate): install a collect_error hook that forwards these diagnostics to the telemetry logging pipeline (at warning level), matching the legacy backend's behaviour.

Protobuf model adjustments

• Re-exported LabelPair from the registry proto module so downstream code can construct/consume label pairs.

Attribution

• The label serialiser and Family are adapted from prometools (MIT OR Apache-2.0); per-definition // Adapted from ... notes were added, per the discussion with @TheJokr. (prometheus-client-derived code remains covered by the existing module-level # License disclaimer.)