Alternative to #2114 that keeps typed metric family descriptors opt-in.

Difference from #2114:

• Keeps the legacy snapshot builders using plain MetricMetadata construction.
• Does not derive counter/info metadata automatically in CounterSnapshot.Builder or InfoSnapshot.Builder.
• Restores the builder fields to private and avoids forcing legacy/deprecated consumers into the typed descriptor path.

The new typed descriptor API remains available for collectors that explicitly opt in to the new behavior.

Related validation:

• test: validate Micrometer vanilla compatibility #2121 validates vanilla Micrometer compatibility against this branch plus fix: restore reserved suffix stripping in PrometheusNaming.sanitizeMetricName() #2094.
• test: validate Micrometer typed-descriptor compatibility #2123 validates a Micrometer branch that explicitly opts in via MetricFamilyDescriptor.

Local validation:

• mise run build