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
• keep OTLP-to-Prometheus naming translation owned by the OTel exporter instead of relying on Prometheus Java naming helpers

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.
• Prometheus Java is still used for the metrics model and exposition, but it only validates and serializes names here; it does not own the OTLP naming mangling.

References

• OTel Prometheus exporter translation strategy spec: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/sdk_exporters/prometheus.md#translation-strategy
• OTel Prometheus/OpenMetrics compatibility normalization rules: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/compatibility/prometheus_and_openmetrics.md#metric-attributes
• Prometheus Go otlptranslator: https://github.com/prometheus/otlptranslator
• Prometheus Go label normalizer used for comparison: https://github.com/prometheus/otlptranslator/blob/main/label_namer.go

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 94.28571% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.77%. Comparing base (327f564) to head (004431f).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
...try/exporter/prometheus/PrometheusUnitsHelper.java	84.61%	3 Missing and 3 partials ⚠️
.../exporter/prometheus/Otel2PrometheusConverter.java	96.22%	1 Missing and 3 partials ⚠️

❌ Your project check has failed because the head coverage (78.48%) is below the target coverage (89.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@              Coverage Diff              @@
##               main    #8346       +/-   ##
=============================================
- Coverage     91.04%   78.77%   -12.27%     
- Complexity     7822     8579      +757     
=============================================
  Files           893     1009      +116     
  Lines         23721    28993     +5272     
  Branches       2364     3599     +1235     
=============================================
+ Hits          21596    22839     +1243     
- Misses         1407     5311     +3904     
- Partials        718      843      +125     

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[psx95]

Also might wanna look at this relevant spec issue: open-telemetry/opentelemetry-specification#5062

[jack-berg]

Some small comments and ideas. Thanks!

[zeitlinger]

Will undraft once the new prom client release has been published.

[zeitlinger]

@psx95 please have another look

[psx95]

Thanks for following up on the updated spec issue, @zeitlinger !

Overall changes LGTM, I just left a couple of small suggestions and mostly verified the translation logic by looking at the tests.

[psx95]

Thanks for addressing the feedback!

[jack-berg]

Re-reviewed. Two more small nits, but still looks good.

[zeitlinger]

@jack-berg done