feat: add normalizeMetricName helper

[zeitlinger]

Summary

Adds a new permissive normalization API on top of #2094.

sanitizeMetricName() keeps the restored compatibility behavior from that PR:
it strips reserved suffixes such as _total, _created, _bucket, and
_info (plus dot variants).

This PR adds normalizeMetricName() as the explicit opt-in for the newer
1.6.x-style permissive behavior:

• keep the original metric name unchanged
• allow reserved suffixes to remain in the normalized name
• still append units when requested

That gives downstream integrations a clear API split:

• sanitizeMetricName(...) for compatibility-preserving suffix stripping
• normalizeMetricName(...) for permissive normalization

Changes

• add PrometheusNaming.normalizeMetricName(String)
• add PrometheusNaming.normalizeMetricName(String, Unit)
• extend PrometheusNamingTest to cover the new helper and its unit overload
• clarify the class-level Javadoc around when to use sanitizeMetricName()
  vs normalizeMetricName()

Why stacked

This is intended to sit on top of #2094.

• fix: restore reserved suffix stripping in PrometheusNaming.sanitizeMetricName() #2094 restores the old sanitizeMetricName() behavior for downstreams that
  relied on suffix stripping
• this PR preserves access to the newer permissive semantics under a separate,
  explicit method name

Test plan

• mise run build
• ./mvnw test -pl prometheus-metrics-model -am -Dtest=PrometheusNamingTest,MetricMetadataTest -Dsurefire.failIfNoSpecifiedTests=false -Dcoverage.skip=true -Dcheckstyle.skip=true