feat(health): SiteAuditBacklog metric (count + age + bytes) (#23 M6)

src/ScadaLink.HealthMonitoring/ISiteHealthCollector.cs

@@ -1,4 +1,5 @@
 using ScadaLink.Commons.Messages.Health;
+using ScadaLink.Commons.Types;
 using ScadaLink.Commons.Types.Enums;
 
 namespace ScadaLink.HealthMonitoring;
@@ -28,6 +29,15 @@ public interface ISiteHealthCollector
     /// <c>AddAuditLogHealthMetricsBridge()</c>.
     /// </summary>
     void IncrementAuditRedactionFailure();
+    /// <summary>
+    /// Audit Log (#23) M6 Bundle E (T6) — replace the latest site-local
+    /// audit-queue backlog snapshot (pending count, oldest pending row,
+    /// on-disk file bytes) used by the next <see cref="CollectReport"/> call.
+    /// Refreshed periodically by the <c>SiteAuditBacklogReporter</c> hosted
+    /// service so each report carries a recent point-in-time view of the
+    /// site→central drain health.
+    /// </summary>
+    void UpdateSiteAuditBacklog(SiteAuditBacklogSnapshot snapshot);
     void UpdateConnectionHealth(string connectionName, ConnectionHealth health);
     void RemoveConnection(string connectionName);
     void UpdateTagResolution(string connectionName, int totalSubscribed, int successfullyResolved);

src/ScadaLink.HealthMonitoring/SiteHealthCollector.cs

@@ -1,5 +1,6 @@
 using System.Collections.Concurrent;
 using ScadaLink.Commons.Messages.Health;
+using ScadaLink.Commons.Types;
 using ScadaLink.Commons.Types.Enums;
 
 namespace ScadaLink.HealthMonitoring;
@@ -15,6 +16,7 @@ public class SiteHealthCollector : ISiteHealthCollector
     private int _deadLetterCount;
     private int _siteAuditWriteFailures;
     private int _auditRedactionFailures;
+    private volatile SiteAuditBacklogSnapshot? _siteAuditBacklog;
     private readonly ConcurrentDictionary<string, ConnectionHealth> _connectionStatuses = new();
     private readonly ConcurrentDictionary<string, TagResolutionStatus> _tagResolutionCounts = new();
     private readonly ConcurrentDictionary<string, string> _connectionEndpoints = new();
@@ -89,6 +91,18 @@ public class SiteHealthCollector : ISiteHealthCollector
         Interlocked.Increment(ref _auditRedactionFailures);
     }
 
+    /// <summary>
+    /// Audit Log (#23) M6 Bundle E (T6) — replace the latest backlog snapshot
+    /// from the site SQLite writer. The field is a single reference write
+    /// (volatile) so the next <see cref="CollectReport"/> sees the most recent
+    /// snapshot — there is no count to reset, the report just carries forward
+    /// whatever was last refreshed.
+    /// </summary>
+    public void UpdateSiteAuditBacklog(SiteAuditBacklogSnapshot snapshot)
+    {
+        _siteAuditBacklog = snapshot ?? throw new ArgumentNullException(nameof(snapshot));
+    }
+
     /// <summary>
     /// Update the health status for a named data connection.
     /// Called by DCL when connection state changes.
@@ -207,6 +221,7 @@ public class SiteHealthCollector : ISiteHealthCollector
             ParkedMessageCount: Interlocked.CompareExchange(ref _parkedMessageCount, 0, 0),
             ClusterNodes: _clusterNodes?.ToList(),
             SiteAuditWriteFailures: siteAuditWriteFailures,
-            AuditRedactionFailure: auditRedactionFailures);
+            AuditRedactionFailure: auditRedactionFailures,
+            SiteAuditBacklog: _siteAuditBacklog);
     }
 }