Reconcile documented and emitted telemetry

[heruan]

Verified against current main. Each item is a place where the README and the code disagree, or where telemetry is emitted with no documentation.

• vaadin.client.rpc.duration is documented at README.md:103 and README.md:220 but is never emitted. It is excluded from ClientMetricNames.ALLOWED and never pushed by VaadinMetricsClient.js. Remove it or implement it. Already tracked separately as vaadin.client.rpc.duration is documented but never emitted #352, so resolve that one and drop this bullet, or close vaadin.client.rpc.duration is documented but never emitted #352 as part of this issue.
• vaadin.observability.traces-session-id (README.md:185) is passed from property to builder to settings, but isTracesSessionId() and ObservationNames.KEY_SESSION_ID are read nowhere. The flag is a no-op and the 4.1 vaadin.session.id span attribute is never emitted. Implement it as a high cardinality span attribute, or delete the flag.
• vaadin.resync and vaadin.ui.access are emitted but missing from the README metrics table. vaadin.resync is registered only by the Boot starter, which is worth stating.
• No meter except vaadin.db.fetch.rows sets a description or baseUnit. Descriptions become # HELP lines in the Prometheus output and appear in Grafana's metric browser, so this is cheap discoverability.
• vaadin.flow.version, a 4.1 span attribute, has no successor. A per request attribute is wasteful, so decide on a one-shot info gauge or a resource attribute instead.

vaadin.data.* and vaadin.observability.data will also need documenting once feat/data-query-metrics merges.