Continue processing by forwarding the request to the next interceptor or the actual service implementation.
IMPORTANT:
You must call proceed exactly once to actually handle the RPC; otherwise, the call will be short-circuited and the service method will not be invoked.
You may transform the incoming request flow (e.g., validation, logging, metering) before passing it to proceed. You may also transform the resulting response Flow before returning it to the framework.
The interceptor must ensure to provide and return a valid number of messages, depending on the method type.
The interceptor must not throw an exception. Use close to terminate the call with an error.