Rebuild Observability Kit on Micrometer

[heruan]

Proposal

Rewrite Observability Kit from scratch on Micrometer (metrics + observation/tracing) instead of OpenTelemetry, as a clean-room replacement for the current 4.x modules.

Proposed modules

Module	Role
observability-kit-micrometer	Framework-agnostic Flow instrumentation (core)
observability-kit-spring	Plain Spring integration (opt-in @Import)
observability-kit-starter	Spring Boot auto-config + starter

Dependency chain: starter → spring → micrometer. Plus IT modules per layer (standalone WAR, plain-Spring WAR, Boot app).

Suggested conventions

• Package com.vaadin.observability.*, next major (5.0).
• Config under vaadin.observability.* (covers metrics and tracing).
• Branded public API (ObservabilityKit, ObservabilityConfig, ObservabilityProperties, …); internals stay unprefixed.

Flow dependency

The core module needs two new listener SPIs landing on Flow main (the rest of the prototype/vaadin-micrometer branch is throwaway):

• feat: add session lock request, acquire, release events flow#24498 — SessionLockListener (session lock request/acquire/release events).
• feat: RPC invocation listener support flow#24499 — RpcInvocationListener (RPC invocation events).

Minimum Flow version = first release containing both; until then, build against a snapshot of those branches.

Open questions

• Replace the OTel modules outright, or ship alongside for a transition period?
• Combine Boot auto-config + starter in one artifact, or split them?
• Exact Spring Boot / Micrometer version baselines.