@Override
    public int requestType() {
        return header.apiKey().id;
    }
Serialize a response into a {@link ByteBuffer}. This is used when the response
will be encapsulated in an {@link EnvelopeResponse}. The buffer will contain
both the serialized {@link ResponseHeader} as well as the bytes from the response.
There is no `size` prefix unlike the output from {@link #buildResponseSend(AbstractResponse)}.

Note that envelope requests are reserved only for APIs which have set the
{@link ApiKeys#forwardable} flag. Notably the `Fetch` API cannot be forwarded,
so we do not lose the benefit of "zero copy" transfers from disk.
clients/src/main/java/org/apache/kafka/common/requests/RequestContext.java