Continue the invocation by forwarding it to the next interceptor or to the underlying transport.
This function is the heart of an interceptor:
It must be called to actually perform the RPC. If you never call proceed, the request is not sent and the call is effectively short-circuited by the interceptor.
You may transform the request flow before passing it to proceed (e.g., logging, retry orchestration, compression, metrics). The returned Flow yields response messages and can also be transformed before being returned to the caller.
Call proceed at most once per intercepted call. Calling it multiple times or after cancellation is not supported.