Fix logging emitter resource packaging and rename to avoid classpath collision

[sarangv]

Fixes #19030 (follow-up).

Description

Follow-up fix for PR #19030 (add defaultMetrics for logging emitter), which introduced metric filtering for the LoggingEmitter. Two issues were discovered during deployment testing in a real Druid cluster:

Fixed resource packaging in processing/pom.xml

The processing/pom.xml already had a <resources> block that only included the Sigar native library directory. When Maven sees any explicit <resources> section, it stops automatically including the default src/main/resources directory. This meant defaultMetrics.json was never packaged into the processing JAR, causing a DruidException (NOT_FOUND) at runtime when LoggingEmitter attempted to load it via getResourceAsStream().

The fix adds <resource><directory>src/main/resources</directory></resource> explicitly alongside the existing Sigar entry.

Renamed defaultMetrics.json to loggingEmitterAllowedMetrics.json

Both the processing module and the extensions-contrib/prometheus-emitter module ship a classpath resource called defaultMetrics.json, but they have completely different schemas:

• Prometheus emitter: objects with dimensions, type, conversionFactor, help fields
• Logging emitter: flat object with metric name keys and empty [] values

Depending on classloader ordering at runtime, LoggingEmitter.class.getClassLoader().getResourceAsStream("defaultMetrics.json") could resolve to the Prometheus emitter's file instead, causing a silent misconfiguration or parse failure. Renaming to loggingEmitterAllowedMetrics.json gives the logging emitter a unique resource name with no collision risk.

Release note

Operators using the logging emitter metric filtering feature introduced in #19030 should note that the bundled default allowlist resource has been renamed from defaultMetrics.json to loggingEmitterAllowedMetrics.json. No configuration changes are needed -- the emitter automatically loads the correct resource. The druid.emitter.logging.allowedMetricsPath property continues to work as documented for custom allowlist files.

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Key changed/added classes in this PR

• LoggingEmitter -- updated resource constant and Javadoc to reference loggingEmitterAllowedMetrics.json
• LoggingEmitterConfig -- updated Javadoc to reference loggingEmitterAllowedMetrics.json
• processing/pom.xml -- added explicit src/main/resources to <resources> block
• processing/src/main/resources/loggingEmitterAllowedMetrics.json -- renamed from defaultMetrics.json
• processing/src/test/resources/loggingEmitterAllowedMetrics.json -- renamed from defaultMetrics.json

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This PR has:

• been self-reviewed.
• added documentation for new or modified features or behaviors.
• a release note entry in the PR description.
• added Javadocs for most classes and all non-trivial methods. Linked related entities via Javadoc links.
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage is met.
• been tested in a test Druid cluster.