Add Datadog APM and metrics hooks to Article module

[devin-ai-integration]

Summary

Adds Datadog observability hooks to the Article module for demo purposes. This is a focused, single-module integration covering both APM tracing and custom business metrics.

What's included:

• Dependencies: dd-trace-api, micrometer-registry-datadog, spring-boot-starter-actuator, spring-boot-starter-aop
• io.spring.observability package:
  • DatadogConfig — Micrometer registry with common tags (service, framework) and @Timed AOP support
  • ArticleMetrics — Custom counters for articles.created, articles.updated, articles.deleted, articles.favorited
• @Trace annotations on ArticleCommandService.createArticle() and updateArticle() for Datadog APM spans
• @Timed annotations on ArticleApi (GET, DELETE) and ArticleFavoriteApi (POST) endpoints
• Spring Security updated to permit unauthenticated access to /actuator/health, /actuator/info, /actuator/metrics/**
• Datadog config in application.properties (disabled by default; set DD_METRICS_ENABLED=true and DD_API_KEY to activate)

How to run with Datadog APM:

java -javaagent:dd-java-agent.jar \
  -Ddd.service=realworld-api \
  -Ddd.env=demo \
  -jar build/libs/realworld-0.0.1-SNAPSHOT.jar

Review & Testing Checklist for Human

• Verify ./gradlew clean test passes (68/68 tests green)
• Confirm Datadog metrics export is disabled by default (DD_METRICS_ENABLED defaults to false)
• Review that actuator endpoints are appropriately permitted in WebSecurityConfig

Notes

• Spotless (./gradlew spotlessCheck) fails on the base branch too with Java 17 due to google-java-format module access restrictions — this is a pre-existing issue, not introduced by this PR.
• The dd-java-agent.jar is a runtime-only component (not bundled). Download it from Datadog releases when running the app.

Link to Devin session: https://app.devin.ai/sessions/95305e1afc44494abc08f411e3a94c30
Requested by: @achalc

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[devin-ai-integration]

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