Add ConnectionRefused as transient network error so gRPC retry works when collector is down (#6259)

Co-authored-by: Rajkumar Rangaraj <[email protected]>

Author: simaoribeiro

src/OpenTelemetry.Exporter.OpenTelemetryProtocol/Implementation/ExportClient/OtlpGrpcExportClient.cs

@@ -158,6 +158,7 @@ private static bool IsTransientNetworkError(HttpRequestException ex)
         return ex.InnerException is System.Net.Sockets.SocketException socketEx
             && (socketEx.SocketErrorCode == System.Net.Sockets.SocketError.TimedOut
                 || socketEx.SocketErrorCode == System.Net.Sockets.SocketError.ConnectionReset
-                || socketEx.SocketErrorCode == System.Net.Sockets.SocketError.HostUnreachable);
+                || socketEx.SocketErrorCode == System.Net.Sockets.SocketError.HostUnreachable
+                || socketEx.SocketErrorCode == System.Net.Sockets.SocketError.ConnectionRefused);
     }
 }