SignalR - Outbound bytes size

Hello,

Do we have any way to understand the bytes size of an outbound message in SignalR?

Maybe some kind of interceptor, that would be able to get payload metadata, after the serialization phase? As I understand Hub Filters are executed before the serialization.
Or maybe there's some kind of SignalR Diagnostic source that I can subscribe.
Another option would be sending preserialized data, so that we can count the bytes before sending it, but as I understand that's not supported.

Thanks in advance.