feat(observability): meter bounded API-key store lookups

src/Core/ZB.MOM.WW.OtOpcUa.Commons/Observability/OtOpcUaTelemetry.cs

@@ -110,6 +110,20 @@ public static class OtOpcUaTelemetry
         Meter.CreateCounter<long>("otopcua.redundancy.primary_gate_denied", unit: "{denial}",
             description: "Operations denied by the Primary gate (site=write|ack|alarm-emit, reason=secondary|detached|role-unknown).");
 
+    /// <summary>
+    ///     API-key store lookups the 03/S18 bound intervened on (outcome=timeout|cache_hit).
+    /// </summary>
+    /// <remarks>
+    ///     One counter with an <c>outcome</c> tag rather than two: <c>timeout</c> is the alarm signal that
+    ///     the ConfigDb is not answering the OPC UA impersonation callback, and <c>cache_hit</c> is the
+    ///     ratio that says whether the TTL is doing anything. A missing <c>cache_hit</c> stream on a busy
+    ///     fused central is how you notice <c>Security:ApiKeys:StoreCacheSeconds</c> was set to 0 by
+    ///     accident. A plain read-through meters neither — the counter names interventions, not traffic.
+    /// </remarks>
+    public static readonly Counter<long> ApiKeyStoreBounded =
+        Meter.CreateCounter<long>("otopcua.opcua.apikey_store.bounded", unit: "{lookup}",
+            description: "API-key store lookups the bound intervened on (outcome=timeout|cache_hit).");
+
     // ---------------- Convenience helpers ----------------
 
     /// <summary>