Record UIDL request and response payload size

Nothing measures how many bytes an interaction puts on the wire. This is the cheapest answer to "is it the network or the server", and it is the number that explains a ComboBox with a ComponentRenderer and a 50-item page feeling slow: the server may report 40 ms while several hundred kilobytes cross the wire per keystroke.

Why this is easy

The kit already owns the filter chain that sees the UIDL body. ResyncDetectionFilter and SpringResyncDetectionFilter buffer the request through CachedBodyHttpServletRequest so ResyncInspector can classify it. A counting HttpServletResponseWrapper beside that gives the response side.

No Flow change is required.

Proposed work

Record two distribution summaries, only for UIDL requests:

• vaadin.uidl.request.bytes
• vaadin.uidl.response.bytes

Tagged by route through RouteTagResolver, so cardinality stays inside the existing routeCardinalityLimit, and by the interaction kind the request interceptor already resolves (poll / navigation / rpc), which separates a heartbeat from a real data load.

Gate it behind a setting in ObservabilitySettings / ObservabilityProperties, consistent with resync and database.

Start with ResyncDetectionFilter, SpringResyncDetectionFilter, CachedBodyHttpServletRequest, and ResyncInspector to trace how UIDL requests are buffered, then inspect the response path and RouteTagResolver. Review ObservabilitySettings and ObservabilityProperties alongside the existing resync and database settings. Done means the two UIDL distribution summaries are recorded with route and interaction-kind tags, subject to the existing cardinality limit and setting gate.