Part 1: Multi-module setup and core Micrometer instrumentation

[heruan]

Sub-issue of #323.

Scope

Establish the Maven multi-module project and the framework-agnostic core module that provides Micrometer-based instrumentation for Vaadin Flow in standalone (non-Spring) deployments.

Deliverables

• Reactor build: parent + BOM-managed Flow / Micrometer / JUnit, Java 21, commercial license.
• observability-kit-micrometer module (package com.vaadin.observability.micrometer).
• Programmatic bootstrap — ObservabilityKit.install(meterRegistry, settings) — storing the registry and settings for standalone use.
• Immutable settings with feature toggles (sessions, UIs, navigation, requests, errors, client, traces) and limits.
• Automatic wiring through Vaadin's service-init SPI, so standalone apps need no manual listener registration.
• First metric: active Vaadin sessions gauge (vaadin.sessions.active).

Out of scope

Spring and Spring Boot integration; the remaining metrics, tracing, and client instrumentation (covered by later parts).