feat(foundations): use foundations-metrics as the macro backend + expose a foundations-metrics facade

[ethanolchik]

PR Description: metrics/runtime

Overview

This PR completes the foundations-metrics crate and integrates it into the foundations telemetry facade. It adds histograms, exemplars, collection, encoding, validation, info metrics, and custom metrics support. The foundations crate gains a new foundations-metrics-backend feature flag (enabled by default) that swaps the legacy prometheus-client/prometools stack for the new implementation while keeping existing call sites source-compatible.

New Metric Types

• Histograms: Added Histogram (classic buckets), TimeHistogram (seconds-scaled), and NativeHistogram (zero-threshold / bucket-factor configuration).
• Exemplars: Added CounterWithExemplar, HistogramWithExemplars, and NativeHistogramWithExemplars for trace-context exemplars.
• Info metrics: Added InfoMetric trait and report_info() for persistent build/process metadata.
• Custom metrics: Exposed EncodeMetric and EncodeMetricValue so users can define their own metric types and register them via NamedMetric and Family.

Collection & Encoding

• Added collect() and CollectionOptions to walk the global registry and emit MetricFamily protobuf messages.
• Implemented encode_to_text (OpenMetrics) and encode_to_protobuf (length-delimited Prometheus protobuf).
• Added Accept-header content negotiation in the telemetry server (see Behaviour Changes).
• Fixed service-label deduplication: the service label is now decided from its first occurrence.
• Media ranges with an unparseable q weight are now dropped rather than treated as q=1.0.

Behaviour Changes

/metrics previously ignored Accept entirely and always returned
application/openmetrics-text; version=1.0.0; charset=utf-8. That is still what every
scraper that sends no Accept, or that accepts text, receives — so the common path is
unchanged. Two cases now behave differently:

• Unsatisfiable Accept returns 406 Not Acceptable instead of text. For example
  Accept: application/json,text/html, or a malformed q on the only supplied range.
  OpenMetrics text is always servable, so a 406 can only mean the client refused text;
  serving text to a client that said it cannot parse text hides a misconfiguration
  instead of surfacing it.
• Protobuf is only offered when it can carry the whole exposition. ExtraProducer
  is a text-only extension point, so while any extra producer is registered the server
  withholds protobuf and serves text. This is all-or-nothing: one registered producer
  disables protobuf for the whole process. A protobuf-only client in that state gets a
  406 rather than a silently truncated exposition.

Protobuf availability is a runtime capability evaluated per scrape, not a setting; no
MetricsSettings field was added. Note that native histograms are not representable in
OpenMetrics text, so a process with both native histograms and an extra producer cannot
export the full set in either format.

Validation & UTF-8

• Added validation pipeline for metric names, label names, and values before encoding.
• Added UTF-8 support with automatic OpenMetrics quoting when needed.

Foundations Integration

• Refactored foundations-macros to register metrics through a backend-agnostic shim, so #[metrics] and #[info_metric] work with either backend.
• Deprecated add_extra_producer / ExtraProducer in favor of structured EncodeMetric registration.
• Preserved the legacy backend behind the disabled foundations-metrics-backend feature.

Crate Readiness

• Added description fields to foundations-metrics and foundations-metrics-registry so they are publishable on crates.io.
• Added crate README.md with migration guide off the telemetry facade.
• Added tests for format negotiation, info metrics, custom metrics, and UTF-8 fixture output.