feat(observability): add diagnostic dashboards (#27)

src/FinalFactory.Rendezvous.Server/Browser/SessionChangeJournal.cs

@@ -143,6 +143,39 @@ internal sealed class SessionChangeJournal
 
     public SessionChangeJournalOptions Options => _options;
 
+    public int ReplayCount
+    {
+        get
+        {
+            lock (_gate)
+            {
+                return _changes.Count;
+            }
+        }
+    }
+
+    public int SubscriberCount
+    {
+        get
+        {
+            lock (_gate)
+            {
+                return _subscribers;
+            }
+        }
+    }
+
+    public int SubscribedTenantCount
+    {
+        get
+        {
+            lock (_gate)
+            {
+                return _subscribersByTenant.Count;
+            }
+        }
+    }
+
     public long CurrentRevision
     {
         get