fix(historian): latch the recorder's dropped-kind Debug log per run; pin the outbox.unreadable gauge

The DroppedUnsupportedKind counter still counts every value; only the Debug
line is latched (cleared by the next captured value), so a Gateway-backend
comms loss no longer logs one line per tag.

Claude-Session: https://claude.ai/code/session_01Ef2ozxS7QPanGT8uoATbTe

src/Server/ZB.MOM.WW.OtOpcUa.Runtime/Historian/ContinuousHistorizationRecorder.cs

@@ -196,6 +196,12 @@ public sealed class ContinuousHistorizationRecorder : ReceiveActor, IWithTimers
     /// the first item that ingests cleanly — see <see cref="IngestAsync"/>.</summary>
     private bool _ingestFaultLogged;
 
+    /// <summary>Whether the current run of unsupported-kind drops has already been logged, so a backend that
+    /// drops a whole class of value (every comms-loss marker on the Gateway backend, say) logs once per run rather
+    /// than once per value. The <see cref="_droppedUnsupportedKind"/> counter still counts every value. Cleared by
+    /// the next value that is captured.</summary>
+    private bool _droppedKindLogged;
+
     /// <summary>Last observed state of <see cref="_captureGate"/>, so a change is logged once rather
     /// than on every value. Seeded open: a gate that starts closed logs its first suppression.</summary>
     private bool _captureGateOpen = true;
@@ -553,10 +559,18 @@ public sealed class ContinuousHistorizationRecorder : ReceiveActor, IWithTimers
             // (e.g. String/Empty on the gateway's numeric-only SQL analog path) — drop and meter. No value
             // content is logged.
             _droppedUnsupportedKind++;
-            _log.Debug("ContinuousHistorization: dropped a value of a kind the historian backend does not support.");
+            if (!_droppedKindLogged)
+            {
+                _droppedKindLogged = true;
+                _log.Debug("ContinuousHistorization: dropped a value of a kind the historian backend does not support. "
+                    + "Further drops are only counted (DroppedUnsupportedKind) until a value is captured.");
+            }
+
             return;
         }
 
+        _droppedKindLogged = false;
+
         var timestampUtc = DateTime.SpecifyKind(msg.TimestampUtc, DateTimeKind.Utc);
 
         entries ??= new List<HistorizationOutboxEntry>();