Part 3: Plain Spring integration (observability-kit-spring)

[heruan]

Sub-issue of #323.

Scope

Add the observability-kit-spring module providing integration for plain (non-Boot) Spring applications.

Deliverables

• observability-kit-spring module depending on the core.
• A Spring @Configuration (opt-in via @Import) that builds settings from vaadin.observability.* properties and registers the instrumentation with the application's MeterRegistry.
• Spring HTTP server observation enrichment — lifts Vaadin request information into the HTTP trace span.

Dependencies

Builds on the core module (#324). The HTTP-observation enrichment relies on the request/tracing instrumentation from Part 2.

Out of scope

Spring Boot auto-configuration (Part 4).