default <T> StreamMessage<T> decode(HttpDecoder<T> decoder, ByteBufAllocator alloc) {
        return DecodedStreamMessage.of(this, decoder, alloc);
    }
Creates a decoded {@link StreamMessage} which is decoded from a stream of {@link HttpObject}s using
the specified {@link HttpDecoder} and {@link ByteBufAllocator}.
core/src/main/java/com/linecorp/armeria/common/HttpMessage.java