Let observer handler work with ExtendedHandler and Filter like Interfaced handler.

[veblush]

Observer handler lacks two good things that Interfaced handler has

• ExtendedHandler

Without ExtendedHandler it's impossible to make observer handler asynchronous.

[ExtendedHandler]
async Task Say()
...

• Filter

Filter is quite useful for interfaced handlers and expect same thing for observer handler.

[Log]
void ChatObserver.Say()
...

[veblush]

Message handlers can have filters like notification handlers, too.