feat: add typed metric family descriptors

[zeitlinger]

Summary

• add a typed MetricFamilyDescriptor registration-time API in prometheus-metrics-model
• reuse the same typed metadata derivation rules as the snapshot builders
• cover the new descriptor API with model tests

Why

This is the end-state API for descriptor-first consumers such as Micrometer.
It removes the need for downstreams to reconstruct MetricMetadata internals
from suffix-heavy name handling at registration time.

Notes

• CounterSnapshot.Builder and InfoSnapshot.Builder continue to own scrape-time typed derivation
• MetricFamilyDescriptor provides the matching registration-time typed entry point
• this is intended to be paired with a downstream Micrometer draft that links against this PR

Validation

• ./mvnw test -pl prometheus-metrics-model -Dtest=MetricMetadataTest,CounterSnapshotTest,GaugeSnapshotTest,InfoSnapshotTest,MetricFamilyDescriptorTest,PrometheusRegistryTest -Dcoverage.skip=true -Dcheckstyle.skip=true
• mise run build