feat: Add OpenMetrics2 configuration support

[jaydeluca]

Part of #1912

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chore: Add OpenMetrics2 configuration support
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[Copilot]

Pull request overview

Adds experimental OpenMetrics 2.0 configuration plumbing to the prometheus-metrics-config module so OM2 feature flags can be set via builder APIs and loaded from properties/env vars (as part of #1912).

Changes:

• Introduce OpenMetrics2Properties with OM2 feature-flag fields, builder, and property loading.
• Wire OM2 properties into PrometheusProperties and PrometheusPropertiesLoader.
• Add/extend unit tests covering defaults, builder usage, enable-all, and property loading/error handling.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
prometheus-metrics-config/src/main/java/io/prometheus/metrics/config/PrometheusPropertiesLoader.java	Loads OpenMetrics2Properties and includes it in the assembled PrometheusProperties.
prometheus-metrics-config/src/main/java/io/prometheus/metrics/config/PrometheusProperties.java	Adds openMetrics2Properties field + getter and builder APIs (enableOpenMetrics2, direct assignment).
prometheus-metrics-config/src/main/java/io/prometheus/metrics/config/OpenMetrics2Properties.java	New OM2 flag container with property parsing and builder (including enableAll()).
prometheus-metrics-config/src/test/java/io/prometheus/metrics/config/PrometheusPropertiesTest.java	Adds tests for OM2 builder patterns, defaults, and loading via PrometheusPropertiesLoader.
prometheus-metrics-config/src/test/java/io/prometheus/metrics/config/OpenMetrics2PropertiesTest.java	New tests for OM2 property loading, invalid values, builder behavior, and defaults.

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[zeitlinger]

Nice clean PR — follows existing patterns well and the test coverage is thorough.

A few things to consider:

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Property prefix: open_metrics2 vs openmetrics2

The existing OTel prefix uses opentelemetry (no separator). open_metrics2 with the underscore looks a bit odd next to it. openmetrics2 would be more consistent — worth a conscious decision since this becomes a public property key.

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Consider marking the API as experimental

Since the OM2 spec is still draft, it might be worth adding a Javadoc note like @apiNote Experimental — subject to change on OpenMetrics2Properties and getOpenMetrics2Properties() so users know the API may change.

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Suffix validation interaction

Opened #1941 to discuss how disableSuffixAppending interacts with _total suffix validation/stripping in PrometheusNaming and Counter.Builder.

[jaydeluca]

Property prefix: open_metrics2 vs openmetrics2
The existing OTel prefix uses opentelemetry (no separator). open_metrics2 with the underscore looks a bit odd next to it. openmetrics2 would be more consistent — worth a conscious decision since this becomes a public property key.

Sure, openmetrics2 does look better IMO, should we go with that?

[zeitlinger]

LGTM — clean flag infrastructure, good test coverage, follows existing patterns.

Two nits (not blocking):

1. Consider adding @apiNote Experimental — subject to change on OpenMetrics2Properties and getOpenMetrics2Properties() for IDE visibility (the class Javadoc mentions it but @apiNote renders more prominently).

2. I'll update #1912 to reflect that disableSuffixAppending is handled separately via #1941 (scrape-time suffix handling) rather than as an OM2 config flag.

[jaydeluca]

we don't use @apiNote anywhere else in this project, and the build doesn't like it:

[javadoc] [ERROR] /home/runner/work/client_java/client_java/prometheus-metrics-config/src/main/java/io/prometheus/metrics/config/OpenMetrics2Properties.java:8: error: unknown tag. Unregistered custom tag?
[javadoc] [ERROR] * @apiNote These properties are experimental and subject to change.
[javadoc] [ERROR] ^