Reconcile documented and emitted telemetry

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 #352, so resolve that one and drop this bullet, or close #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.

Start with the telemetry tables in README.md and compare them with ClientMetricNames.ALLOWED, VaadinMetricsClient.js, the traces-session-id property/builder/settings path, and the Boot starter registration. Review isTracesSessionId(), ObservationNames.KEY_SESSION_ID, and the existing vaadin.db.fetch.rows description and baseUnit. Done means each listed discrepancy has an implemented or removed behavior, documented metrics and attributes match emissions, and the remaining design choices are resolved.