Add the message body back to the MessageFailed event when non-binary & under LOH limit (#4475)

Author: DavidBoike

src/ServiceControl/Recoverability/ExternalIntegration/MessageFailedConverter.cs

@@ -75,6 +75,10 @@ static string GetBody(FailedMessage.ProcessingAttempt last)
             {
                 return (string)body;
             }
+            else if (last.MessageMetadata.TryGetValue("MsgFullText", out body))
+            {
+                return (string)body;
+            }
 
             return last.Body;
         }