feat(metrics): add protobuf and OpenMetrics text encoders and collection

[ethanolchik]

Overview

This PR completes the path from registered metrics to Prometheus-compatible wire output:

1. Collect registered metrics into the canonical protobuf MetricFamily model.
2. Encode those families as length-delimited Prometheus protobuf or OpenMetrics text.

The public API now exposes collect, CollectionOptions, ServiceNameFormat, encode_to_protobuf, and encode_to_text.

Collection

collect iterates over the global registry and encodes each registered metric into one or more MetricFamily values.

CollectionOptions controls whether optional metrics are included and how a service name is represented:

• MetricPrefix prefixes metric family names, except metrics registered as unprefixed.
• LabelWithName adds the service name as a label to every row.

The resulting Vec<MetricFamily> provides a common boundary between metric implementations and wire encoders.

Encoding

encode_to_protobuf serialises metric families as concatenated, length-delimited Prometheus protobuf messages. This preserves fields that OpenMetrics text cannot fully represent, including native histogram data.

encode_to_text supports:

• Counters
• Gauges
• GaugeHistograms
• Summaries
• Untyped metrics
• Classic histograms

Info metrics have no dedicated metric type; they are ordinary gauge families valued 1 that carry their fields as labels. Tests pin this representation in both formats (the family name is unchanged (for example, build_info), the protobuf type is gauge, and text emits # TYPE build_info gauge) so that the info-metric layer added in a later PR reproduces the legacy wire output exactly. This PR adds no way to define or report an info metric.

The text encoder emits # HELP, # TYPE, and # UNIT metadata and terminates output with # EOF. Empty # HELP lines are omitted from output.

Histogram encoding:

• Maps the terminal f64::MAX bucket to +Inf.
• Adds a terminal +Inf bucket when missing.
• Preserves classic histogram bucket exemplars.
• Skips native-only histogram rows.
• Skips histogram rows with positive floating-point counts.

The encoder also handles metric and exemplar timestamps, escapes help text and label values, and emits NaN, +Inf, and -Inf using OpenMetrics syntax.

Summary and GaugeHistogram families are fully supported. Protobuf passes them through unchanged. In text, summaries emit their quantile rows plus _sum and _count; gauge histograms emit _bucket rows plus _gsum and _gcount.

For text output, families without a non-empty name or recognised type, and rows missing data for their declared type, are skipped and reported through the non-fatal collection error hook.