@Override
        public void onDrainStart(ChannelHandlerContext ctx) {
            goAway(ctx, Integer.MAX_VALUE);
            ctx.flush();
        }
Send GOAWAY frame with stream ID 2^31-1 to signal clients that shutdown is imminent,
but still accept in flight streams.
core/src/main/java/com/linecorp/armeria/server/Http2ServerConnectionHandler.java