feat(metrics): add support for classic, time, and native histograms.

[ethanolchik]

Overview

Adds classic, time, and native histogram metric types to foundations-metrics, including protobuf encoding, constructors, family support, TimeHistogram snapshots, and timer behaviour.

Changes

Classic histograms

Adds an owned Histogram backed by shared, mutex-protected storage.

• Histogram::new accepts inclusive bucket bounds and appends the legacy terminal f64::MAX bucket.
• Cloned handles share observations.
• observe updates the sample sum, sample count, and the first matching bucket.
• Sorted buckets use a binary search while unsorted buckets retain the legacy linear-search behaviour.
• NaN observations preserve legacy behaviour by updating sum/count without incrementing a bucket.
• Protobuf output contains a histogram row with cumulative bucket counts, sample count, and sample sum.

Time histograms

Adds the lock-free TimeHistogram and HistogramTimer APIs previously provided by prometools.

• Observations are stored atomically in nanoseconds and exported in seconds.
• start_timer records elapsed time when the timer is stopped or dropped.
• Timers support pause, resume, stop_and_record, and stop_and_discard.
• A timer records at most once.
• TimeHistogram::snapshot exposes the current sum, count, and non-cumulative buckets through HistogramSnapshot.

Histogram construction

Adds HistogramBuilder as a MetricConstructor for both Histogram and TimeHistogram.

This allows configured histograms to be created by Family and by generated metric constructors while preserving the existing #[ctor = HistogramBuilder { ... }] shape.

Native histograms

Adds NativeHistogram, backed by prometheus-client 0.25's native histogram implementation.

• NativeHistogram::observe records values into sparse exponential buckets.
• Cloned handles share the same storage.
• NativeHistogramBuilder configures bucket growth factor, zero-bucket threshold, and the best-effort maximum number of sparse buckets.
• The builder implements MetricConstructor, allowing native histograms to be used in labelled Family metrics.
• Native histogram encoding delegates to prometheus-client because its sparse bucket state is private.
• The resulting protobuf families are converted into the canonical foundations-metrics-registry model, including sample count/sum, schema, zero bucket, positive and negative spans, and bucket deltas.
• Encoding errors are reported through the existing non-fatal collection diagnostic hook and skip only the affected histogram.

Public API

Exports the following from foundations-metrics:

• Histogram
• HistogramBuilder
• HistogramSnapshot
• HistogramTimer
• TimeHistogram
• NativeHistogram
• NativeHistogramBuilder

The registry crate also re-exports the protobuf Bucket type needed by histogram encoders.