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

tests/ScadaLink.SiteRuntime.Tests/Actors/DeploymentManagerRedeployTests.cs

@@ -71,6 +71,7 @@ public class DeploymentManagerRedeployTests : TestKit, IDisposable
         public void IncrementDeadLetter() { }
         public void IncrementSiteAuditWriteFailures() { }
         public void IncrementAuditRedactionFailure() { }
+        public void UpdateSiteAuditBacklog(ScadaLink.Commons.Types.SiteAuditBacklogSnapshot snapshot) { }
         public void UpdateConnectionHealth(string connectionName, ConnectionHealth health) { }
         public void RemoveConnection(string connectionName) { }
         public void UpdateTagResolution(string connectionName, int totalSubscribed, int successfullyResolved) { }