feat(java): add spring-boot-microservices-demo with distributed tracing

[Vinu2111]

What does this PR add?

A new Java example demonstrating distributed tracing across two Spring Boot
microservices using OpenTelemetry auto-instrumentation and SigNoz.

Services

• order-service (port 8081) — accepts orders, calls inventory-service
• inventory-service (port 8082) — checks stock availability

What you can observe in SigNoz

• Single distributed trace spanning both services
• Auto-instrumented HTTP spans (inbound + outbound via RestClient)
• Manual spans via @WithSpan (order.process, inventory.check)
• W3C trace context propagation across service boundaries
• Error traces for out-of-stock (400) and not-found (404) scenarios
• JVM metrics and correlated logs — zero custom metric code needed

Stack

• Java 21, Spring Boot 3.5.0
• OTel Java Agent 2.26.0 (auto-instrumentation only)
• Maven, Docker Compose

Testing done

• Happy path ✅
• Out of stock (400) ✅
• Product not found (404) ✅
• Verified traces visible in local SigNoz instance ✅

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