fix: Preserve original metric names in OM2 output

[zeitlinger]

Refs #1954.

Summary

This fixes OM2 name preservation in the text writer.

• Use original metric metadata names in OM2 output instead of OM1 exposition base names.
• Preserve original names for default classic histogram output instead of delegating that path to OM1.
• Keep the OM2 _info exception for info metrics.
• Add builder-based regression coverage for counters and classic/native histograms with .unit(...).

Why

openmetrics2.enabled=true is documented as preserving metric names as written by the application. The writer still used OM1-derived names in several paths, and classic histograms delegated to the OM1 writer, so .unit(...) could still leak unit-suffixed names into OM2 output.

Smoking gun

prometheus-metrics-core/src/test/java/io/prometheus/metrics/core/metrics/OpenMetrics2TextFormatWriterTest.java
adds counterPreservesOriginalNameWhenUnitIsConfigured().

That test builds a real counter through the public builder API:

Counter.builder().name("my_counter").unit(Unit.SECONDS)

It then asserts that OM1 still emits the legacy name my_counter_seconds_total, while OM2 emits
my_counter and does not contain my_counter_seconds.

I verified this test fails against pre-fix origin/main when only the test is applied:

OpenMetrics2TextFormatWriterTest.counterPreservesOriginalNameWhenUnitIsConfigured

Expecting actual:
  "# TYPE my_counter_seconds counter
# UNIT my_counter_seconds seconds
# HELP my_counter_seconds Test counter
my_counter_seconds{method="GET"} 42.0 st@...
# EOF
"
to contain:
  "# TYPE my_counter counter
"

That is the bug this PR fixes: OM2 was still using the OM1 unit-suffixed metric name.

Validation

• ./mvnw test -pl prometheus-metrics-core,prometheus-metrics-exposition-textformats -am -Dtest=OpenMetrics2TextFormatWriterTest -Dcoverage.skip=true -Dcheckstyle.skip=true -Dsurefire.failIfNoSpecifiedTests=false
• mise run build
• mise run lint
• git diff --check