Blocked by open-telemetry#8346

Summary

Stacked on top of open-telemetry#8346.

This refactors the Prometheus exporter to use the new typed client_java
family descriptor API where the translation strategy already matches typed
Prometheus family semantics.

Depends on:

• Add Prometheus translation strategy support open-telemetry/opentelemetry-java#8346
• feat: add typed metric family descriptors prometheus/client_java#2114

What changed

• use MetricFamilyDescriptor for target_info
• use MetricFamilyDescriptor when deriving metadata for:
  • UNDERSCORE_ESCAPING_WITH_SUFFIXES
  • NO_UTF8_ESCAPING_WITH_SUFFIXES
• keep the legacy direct MetricMetadata construction paths for the
  no-suffix translation modes for now
• update the metadata translation test to reflect the typed descriptor-backed
  metadata shape for UTF-8 counter translation

Why

#8346 expands translation-strategy support, but the exporter still manually
constructs Prometheus MetricMetadata in the conversion path.

With client_java#2114, the Prometheus-semantic translation modes can rely on
upstream typed metadata derivation instead of rebuilding those rules locally.

The no-suffix translation modes are intentionally left on the compatibility
path for now because they do not map cleanly to typed counter semantics.

Validation

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

./gradlew --no-build-cache \
  -I /tmp/otel-prom-local.init.gradle \
  :exporters:prometheus:test \
  --tests io.opentelemetry.exporter.prometheus.Otel2PrometheusConverterTest \
  --tests io.opentelemetry.exporter.prometheus.PrometheusMetricReaderTest \
  --tests io.opentelemetry.exporter.prometheus.PrometheusHttpServerTest

Notes

• This is a stacked draft for review of the direction, not something expected
  to pass upstream CI before the client_java API lands in a published
  dependency.