Add Prometheus translation strategy support

[zeitlinger]

Summary

• add TranslationStrategy support to the Prometheus exporter builder and declarative config
• upgrade Prometheus client_java to 1.6.1, which provides the released naming support this needs
• preserve existing content negotiation behavior, while enabling non-default strategies to use OM2 name-preservation semantics
• align underscore-escaping label-name normalization with the OTel Prometheus compatibility rules / prometheus/otlptranslator behavior for invalid characters, repeated underscores, and digit-leading labels

Notes

• For non-default translation strategies, this intentionally uses only OM2 name-preservation semantics.
• No other OM2 features are enabled.
• Content negotiation remains unchanged, so OpenMetrics responses still use the legacy OM1 content type by design.
• The behavior change from the new Prometheus release is that metric names can now be passed through as-is for the relevant strategies.
• prometheus/otlptranslator preserves labels normalized to __...__; Prometheus Java rejects user labels starting with __, so those labels are collapsed to a valid single-underscore form instead.

Test plan

• ./gradlew :exporters:prometheus:test --tests io.opentelemetry.exporter.prometheus.PrometheusHttpServerTest.fetchOpenMetrics --tests io.opentelemetry.exporter.prometheus.PrometheusHttpServerTest.fetchOpenMetrics_translationStrategyEnablesOm2 --tests io.opentelemetry.exporter.prometheus.PrometheusMetricReaderTest --tests io.opentelemetry.exporter.prometheus.Otel2PrometheusConverterTest --tests io.opentelemetry.exporter.prometheus.internal.PrometheusMetricReaderProviderTest :sdk-extensions:declarative-config:test --tests io.opentelemetry.sdk.autoconfigure.declarativeconfig.MetricReaderFactoryTest.create_PullPrometheusConfigured

Resolves #8195

[codecov]

Codecov Report

❌ Patch coverage is 88.11881% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.29%. Comparing base (df8063b) to head (2cc649c).

Files with missing lines	Patch %	Lines
.../exporter/prometheus/Otel2PrometheusConverter.java	90.00%	2 Missing and 3 partials ⚠️
...ometheus/internal/PrometheusComponentProvider.java	55.55%	3 Missing and 1 partial ⚠️
...try/exporter/prometheus/PrometheusUnitsHelper.java	85.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #8346      +/-   ##
============================================
- Coverage     90.31%   90.29%   -0.03%     
- Complexity     7725     7762      +37     
============================================
  Files           850      851       +1     
  Lines         23259    23352      +93     
  Branches       2364     2377      +13     
============================================
+ Hits          21007    21085      +78     
- Misses         1528     1537       +9     
- Partials        724      730       +6     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.