Part 2: Complete core metrics, tracing, and client instrumentation

[heruan]

Sub-issue of #323.

Scope

Complete the framework-agnostic core (observability-kit-micrometer) with the full set of metrics, observation-based tracing, and client-side instrumentation.

Deliverables

• Metric binders for UIs, navigation (with a route-cardinality cap), and requests/errors.
• Session-lock contention metrics and client/RPC interaction metrics.
• Observation-based tracing, propagating context across UI.access(...) thread boundaries.
• Browser→server client metrics collection with per-session rate limiting.
• Extend the service-init wiring so each feature activates behind its settings toggle.

Dependencies

Requires upstream Flow APIs vaadin/flow#24498 (session-lock events) and vaadin/flow#24499 (RPC invocation events) to be merged/released. The remaining instrumentation does not depend on them; the Spring/Boot parts (3–4) are independent of these PRs.

Out of scope

Spring and Spring Boot integration.