how to parse the request header information for http 2.0?

As shown below，I want to parse the header information for http 2.0，It doesn't feel like it's working。

Can you help me see why？

`
server {
listen 8081 http2;

    set $target_url  '127.0.0.1:8098' ;


    location / {
        content_by_lua_block{
            ngx.log(ngx.ERR, 'access ')
            local headers = ngx.req.get_headers()
            for k, v in pairs(headers) do
                ngx.log(ngx.ERR,"headerK = "..k..' value = '..v)
                ngx.say("headerK = "..k..' value = '..v)
            end
        }
        grpc_pass   grpc://$target_url;
        
    }
}

`

The example uses an nginx server block with an HTTP/2 listener and calls ngx.req.get_headers() inside content_by_lua_block; start by reproducing the configuration and inspecting the ngx.log output. Done means identifying why the request headers are not available as expected and confirming the behavior with a focused reproduction.