BE: fix prometheus endpoint and warn on label duplicates

[germanosin]

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[Copilot]

Pull request overview

This PR fixes the Prometheus metrics endpoint by correcting the content type from OpenMetrics to Prometheus format and adds defensive logic to prevent label duplication with appropriate warnings.

• Changed the HTTP response content type from OpenMetricsTextFormatWriter.CONTENT_TYPE to PrometheusTextFormatWriter.CONTENT_TYPE for the /metrics endpoint
• Added duplicate label detection in MetricsUtils.extendLabels() that logs a warning and preserves the original label value when duplicates are encountered
• Introduced integration test for the Prometheus metrics endpoint with JMX-enabled Kafka container configuration

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
api/src/main/java/io/kafbat/ui/service/metrics/prometheus/PrometheusMetricsExposer.java	Changed content type header from OpenMetrics to Prometheus format
api/src/main/java/io/kafbat/ui/util/MetricsUtils.java	Added label duplication check with warning log before extending labels
api/src/test/java/io/kafbat/ui/controller/PrometheusExposeControllerTest.java	Added integration test for Prometheus metrics endpoint
api/src/test/java/io/kafbat/ui/AbstractIntegrationTest.java	Configured JMX port exposure and Prometheus metrics settings for test cluster

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