feat: use typed Prometheus family descriptors

[zeitlinger]

Summary

This is a cleaner follow-up to the Prometheus metadata compatibility work for Micrometer.

Instead of reconstructing MetricMetadata from Micrometer-side suffix logic, this change switches the Prometheus registry integration to use the new typed registration descriptor API from client_java.

Depends on:

• feat: add typed metric family descriptors prometheus/client_java#2114

Related context:

• Fix Prometheus registry metadata for prom-client-java upgrades #7460

What changed

• replace Micrometer's stored registration metadata shape with MetricFamilyDescriptor
• remove Micrometer-side MetricMetadata reconstruction for registration metadata
• derive registration metadata directly from the typed descriptor API
• keep scrape-time snapshot creation on typed snapshot builders
• add/update registration collision coverage in Prometheus registry tests

Why

Micrometer needs registration-time family metadata for collision validation, but the old shape forced Micrometer to duplicate Prometheus naming derivation details.

With client_java exposing typed family descriptors, Micrometer can now treat Prometheus metadata as an upstream concern instead of rebuilding it locally.

Validation

Tested locally against a locally installed prometheus/client_java 1.6.2-SNAPSHOT containing the descriptor API from prometheus/client_java#2114:

./gradlew --no-build-cache \
  -I /tmp/micrometer-prom-local.init.gradle \
  :micrometer-registry-prometheus:test \
  --tests io.micrometer.prometheusmetrics.MicrometerCollectorTest \
  --tests io.micrometer.prometheusmetrics.PrometheusMeterRegistryTest

Notes

• This draft PR is expected to need prometheus/client_java#2114 first.
• Until that API is released, this branch is primarily for review of the end-state design.