feat(metrics): introduce foundations-metrics with scalar metrics

[ethanolchik]

Overview

This pull request introduces the new foundations-metrics crate, which provides the initial scalar metric types and converts their values into the Prometheus protobuf data model.

It builds on foundations-metrics-registry, which continues to own the stable process-global registry and protobuf types. This separation allows metric implementations to evolve without destabilising the registry shared across Foundations versions.

New Crate: foundations-metrics

• Added the crate's public API, module structure, and documentation.
• Re-exported register, EncodeMetric, IntoMetrics, MetricFamily, and RegistrationMetadata, allowing callers to use the higher-level crate without importing registry APIs separately.

Scalar Metrics

• Implemented Counter with u64 and f64 support.
• Implemented Gauge with i64 (since prometheus_client v0.19.0), u64, and f64 support.
• Implemented RangeGauge, which exports the current, minimum, and maximum values observed since the previous collection.
• Implemented GaugeGuard, which increments a gauge when created and decrements it when dropped.
• Scalar metrics own their atomic storage rather than wrapping prometheus-client. The spec allowed either approach; owned storage keeps the scalar API independent of upstream implementation details and defers the dependency until it is needed for native histograms, though it is still included in foundations-metrics/Cargo.toml.
• Cloned metrics share their underlying atomic storage through Arc, preserving inexpensive handle semantics.
• Built-in storage intentionally focuses on the 64-bit numeric types currently used by Foundations. The public CounterAtomic and GaugeAtomic traits allow additional storage implementations to be added without changing the metric API.

Registration and Encoding

• Introduced NamedMetric, which associates metric storage with its exported name and help text. Metric handles intentionally contain only state because the metrics macro already owns registration metadata.
• Added the internal EncodeMetricValue trait. Storage types encode protobuf families with relative names, while NamedMetric applies the registered base name and help text.
• Scalar metrics emit an empty relative suffix. RangeGauge emits "", _min, and _max, allowing it to compose with NamedMetric and future labelled families without special handling.
• Counter families are encoded without automatically appending _total, preserving existing Foundations output behaviour.

Workspace Updates

• Registered foundations-metrics as a workspace member and workspace dependency.
• Updated foundations-metrics-registry to export the protobuf Counter and Gauge message types.