feat(telemetry): correct ledger-close-time derivation on dashboards

The "ledger close time" was mis-derived and partly un-queryable:

- Build vs Close Duration derived close time from the consensus.ledger_close
  span, which only wraps the onClose() prologue (~0.8ms live) — not the close.
  Repoint the close series to consensus.round (full round, live P95 ~4.8s).
- The network close-time value (close_time) lived only as a span attribute,
  un-queryable in Prometheus and unfit as a spanmetrics label (monotonic
  timestamp -> unbounded cardinality). Expose it as last_close_time on the
  existing server_info observable gauge (native gauge, no new instrument).
- Add a "Ledger Close Interval & Age" panel to ledger-operations and
  node-health: interval = 1/rate(ledgers_closed_total) (counter-based,
  scrape-independent); age = time() - (last_close_time + epoch_offset).

A gauge delta is deliberately NOT used for the interval — a timestamp gauge's
delta aliases to the scrape period, not the close cadence (verified live).
Guardrail comments in both collector configs record why close_time must never
become a spanmetrics dimension. Docs (09-reference) and the operator runbook
updated to match.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/xrpld/telemetry/MetricsRegistry.cpp

@@ -848,6 +848,22 @@ MetricsRegistry::registerServerInfoGauge()
                         "last_close_converge_time_ms",
                         static_cast<int64_t>(consensusInfo["previous_mseconds"].asUInt()));
                 }
+
+                // Network close time of the last closed ledger, as NetClock
+                // seconds since the Ripple epoch (2000-01-01). Unlike a span
+                // timestamp, a gauge value survives as a queryable time series,
+                // so dashboards can show last-close age (staleness) via
+                // now - value. The close interval comes from the
+                // ledgers_closed_total counter, not a delta of this gauge
+                // (a timestamp gauge's delta aliases to the scrape period).
+                // Skip until a ledger has closed.
+                if (auto const closed = app.getLedgerMaster().getClosedLedger())
+                {
+                    observe(
+                        "last_close_time",
+                        static_cast<int64_t>(
+                            closed->header().closeTime.time_since_epoch().count()));
+                }
             }
             catch (...)  // NOLINT(bugprone-empty-catch)
             {