support environment variables in properties

[zeitlinger]

Summary

No Breaking Changes. This is fully backward compatible:

• ✅ Existing camelCase properties in files still work
• ✅ Existing system properties still work
• ✅ Property precedence maintains existing behavior (external > system > file)
• ✅ New environment variable layer added between external and system properties

Migration Guide

No migration required. However, to adopt the new features:

1. To use environment variables: Convert property names to uppercase with underscores

   • io.prometheus.exporter.httpServer.port → IO_PROMETHEUS_EXPORTER_HTTP_SERVER_PORT

2. To use snake_case (recommended): Update properties files

   • io.prometheus.metrics.exemplarsEnabled → io.prometheus.metrics.exemplars_enabled
   • Old camelCase names continue to work

Notes

• adjust new property added in Add GC duration histogram #1738

[Copilot]

Pull request overview

Adds an environment-variable configuration layer and standardizes property naming to snake_case while preserving backward compatibility via key normalization.

Changes:

• Introduces PropertySource to unify external, environment, and “regular” (system/file/classpath) properties with defined precedence.
• Adds environment-variable loading (IO_PROMETHEUS_*) and camelCase→snake_case normalization for file/system/external keys.
• Updates config docs, test resources, and tests to use the new snake_case property names and new loader APIs.

Reviewed changes

Copilot reviewed 23 out of 23 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
prometheus-metrics-config/src/main/java/io/prometheus/metrics/config/PropertySource.java	New abstraction for multi-source property lookup with precedence and key transformations.
prometheus-metrics-config/src/main/java/io/prometheus/metrics/config/PrometheusPropertiesLoader.java	Loads properties from file/classpath/system, adds env var layer, normalizes keys, validates leftovers.
prometheus-metrics-config/src/main/java/io/prometheus/metrics/config/Util.java	Refactors property lookup helpers to read from PropertySource and improve key formatting in errors.
prometheus-metrics-config/src/main/java/io/prometheus/metrics/config/MetricsProperties.java	Migrates internal property-name constants to snake_case and updates loading/validation paths.
prometheus-metrics-config/src/main/java/io/prometheus/metrics/config/ExporterProperties.java	Switches exporter property keys to snake_case and loads via PropertySource.
prometheus-metrics-config/src/main/java/io/prometheus/metrics/config/ExporterHttpServerProperties.java	Updates prefix/key naming to snake_case and loads via PropertySource.
prometheus-metrics-config/src/main/java/io/prometheus/metrics/config/ExporterFilterProperties.java	Renames filter keys to snake_case and updates loader wiring.
prometheus-metrics-config/src/main/java/io/prometheus/metrics/config/ExporterOpenTelemetryProperties.java	Renames OTel keys to snake_case and updates loader wiring.
prometheus-metrics-config/src/main/java/io/prometheus/metrics/config/ExporterPushgatewayProperties.java	Renames pushgateway keys to snake_case and updates loader wiring.
prometheus-metrics-config/src/main/java/io/prometheus/metrics/config/ExemplarsProperties.java	Renames exemplars keys to snake_case and loads via PropertySource.
prometheus-metrics-config/src/test/java/io/prometheus/metrics/config/UtilTest.java	Updates tests to use PropertySource-based APIs.
prometheus-metrics-config/src/test/java/io/prometheus/metrics/config/PrometheusPropertiesTest.java	Updates loading tests to use PropertySource and asserts properties are consumed.
prometheus-metrics-config/src/test/java/io/prometheus/metrics/config/PrometheusPropertiesLoaderTest.java	Updates loader tests to snake_case keys.
prometheus-metrics-config/src/test/java/io/prometheus/metrics/config/MetricsPropertiesTest.java	Updates expected exception messages and property keys to snake_case.
prometheus-metrics-config/src/test/java/io/prometheus/metrics/config/ExporterPropertiesTest.java	Updates keys and loader helper to use PropertySource.
prometheus-metrics-config/src/test/java/io/prometheus/metrics/config/ExporterHttpServerPropertiesTest.java	Updates keys and loader helper to use PropertySource.
prometheus-metrics-config/src/test/java/io/prometheus/metrics/config/ExporterFilterPropertiesTest.java	Updates keys and loader helper to use PropertySource.
prometheus-metrics-config/src/test/java/io/prometheus/metrics/config/ExporterOpenTelemetryPropertiesTest.java	Updates keys and loader helper to use PropertySource.
prometheus-metrics-config/src/test/java/io/prometheus/metrics/config/ExporterPushgatewayPropertiesTest.java	Updates loader helper to use PropertySource.
prometheus-metrics-config/src/test/java/io/prometheus/metrics/config/ExemplarsPropertiesTest.java	Updates keys and loader helper to use PropertySource.
prometheus-metrics-config/src/test/resources/prometheus.properties	Updates test properties file to snake_case keys.
prometheus-metrics-config/src/test/resources/emptyUpperBounds.properties	Updates test properties key to snake_case.
docs/content/config/config.md	Documents env var support, precedence, and snake_case naming; updates examples/tables.

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